McMullen, Conley K.

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SEED GERMINATION STUDIES 
OF SELECTED GALAPAGOS ISLANDS ANGIOSPERMS 
by 
Conley K. McMullen 
Department of Botany. University of Maryland. College Park. Maryland 20742 
Many problems in the evolution of flowering plants are posed by immigration to oceanic islands. 
Obstacles that must be overcome by a plant when colonizing such islands include seed transport, 
germination, establishment, and reproduction. The first of these, seed transport, has been dealt with 
extensively in the literature, both for oceanic islands in general, and for the Galapagos Islands specifically 
(Ridley, 1930; Carlquist, 1974). The most recent figures for the Galapagos would indicate that birds have 
been the most important dispersal agents of flowering plants representing 48.6% of the dispersal events, 
followed by man (38.8%), oceanic drift (7.0%), and wind (5.6%) (Porter, 1983). 
This paper reports on viability and germination tests that were performed on seeds of several species of 
flowering plants of Isla Santa Cruz. The species chosen for these experiments were among those 
comprising a larger study dealing with the reproductive biology of Galapagos Islands angiosperms. The 
objectives of these tests were to determine: I) seed germination ability; 2) the presence or absence of seed 
dormancy; and 3) what correlations exist between seed dormancy and plant resident status, habitat and 
dispersal mechanism. 
The seeds used for the viability tests were obtained from plants located in five of the seven major 
vegetation zones on the southern slope of Santa Cruz (Wiggins and Porter, 1971; van der Werff, 1979). The 
locations of the study quadrats by vegetation zone were as follows: I) Arid Zone - near Darwin Station; 
2) Transition Zone - 3.5 km N Puerto Ayora near road; 3) Zanthoxylum Zone - near Santa Rosa; 4) 
Scalesia Zone - near Los Gemelos and 5) Pampa Zone - 3 km N Media Luna. The seeds were tested for 
viability using a 0.1 % solution of 2,3,5 Triphenyl Tetrazolium Chloride (TIC) which acts as an indicator 
or respiration in the embryo. All of the species included in this study showed positive TIC results 
indicating that the seeds were viable and possessed the potential for germination. 
For the germination tests seeds of each species were divided into two groups. The first group was subjected 
to five minutes exposure in a 2.6% solution of sodium hypochlorite. This treatment was performed to 
sterilize the seed coat and in so doing destroy any fungi that might be present. This treatment was found 
necessary after preliminary tests involving the seeds of several other species had to be terminated due to 
fungal damage. The second group of seeds was subjected to the same solution, but for one hour duration. 
This second treatment was mentioned by Rick and Bowman (1961) as being quite effective in breaking 
dormancy in Lycopersicon cheesmanii var. minor. The type of dormancy that would be affected by this 
kind of treatment is one in which there is a structural barrier to germination which might be broken by 
removal of part of the seed coat by the solution. The seeds were then rinsed very thoroughly in water and 
placed on moist filter paper in petri dishes and observed for a period of21 days. The paper was kept moist 
and the seeds were counted and removed as they germinated. As a precaution against contamination the 
dishes and water were sterilised by autoclaving. For Croton scouleri var. scouleri and Scutia spicata var. 
pauciflora only enough seeds were available for the control treatment. 
The results of the germination tests are shown in Table 1. The percentage of seeds germinated for each 
exposure period as well as the number of seeds tested (in parentheses) is given. It was arbitrarily decided 
that seed dormancy would be indicated for those plants in which there was less than 25% germination 
under the five minute (control) exposure. Using this measure 51.9% of the species tested showed seed 
dormancy. None of these species showed a significant increase in seed germination after the one hour 
exposure period. This would seem to indicate either that the dormancy type is not one of a simple 
mechanical barrier brought about by the seed coat, but perhaps a dormancy requiring some other 
treatment, for example the removal of a certain chemical from within the seed'Coat; or that the seed coats 
required an even longer exposure to the sodium hypochlorite solution. This latter possibility is supported 
by the observation that several of the species showing dormancy seemed to have seeds with thick, tough 
seed coats. Examples of this are seen especially well in Tournefortia rufo-sericea, Croton scouleri var. 
scouleri, Cassia occidentialis, and C1erodendrum molle var. molle. This contrasts with several of those 
plants not showing seed dormancy, such as Justicia galapagana. Adenostemma platyphyllum, and Jaegeria 
gracilis. which have seed coats that are notas thick and that showed a significant drop in germination after 
exposure for an hour to the solution indicating that some of the embryos may have been destroyed. 
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Noticias de Galapagos, vol. 44 1986
Table I also includes the locations by vegetation zones of the plants tested. The arid and transition zones 
are considered part of the basically arid ecological zone, while the Zanthoxylum. Scalesia. and pampa 
zones belong in the basically mesic ecological zone (Johnson and Raven, 1973). Also given is the resident 
status and supposed disseminule dispersal mechanism for each of the plants (Porter, 1983). This 
information was included to determine what correlations existing between these factors and seed 
dormancy. 
Seed dormancy was found in 63.6% of the endemics, 60.0% of the natives, and only 36.4% of the non-
indigenous (introduced weeds and cultivated escapes) species. If the species that were tested are divided 
into just two groups, indigenous and non-indigenous, 62.5% of the former show seed dormancy while only 
36.4% of the latter show this characteristic. This would indicate that the non-indigenous plants are better 
able to immediately exploit any habitat that might open up, whereas the endemics and natives show a 
"strategy" of waiting for more favorable conditions before germinating. It is worth noting that the 
majority of introduced weeds showing greater than 50% germination have a high seed set potential which 
could compensate in part for a high mortality that might result from more immediate or almost total seed 
germination in a less than hospitable environment. Unfortunately, these factors also help to explain why 
some of the introduced weeds and cultivated escapes on the Islands are successful in advancing into areas 
that are temporarily exposed due to factors such as fires and pasture use. These plants are pre-adapted to 
making a quick start in these disturbed locations and in tum slowing or halting the return of the native 
vegetation. 
When ecological zones are considered 66.7% of the species in the basically arid zone show seed dormancy 
while 33.3% of those in the basically mesic zone show dormancy. The explanation for this seems 
straightforward enough in that plants which would be favored to survive in the dry or xeric conditions 
represented in the transition zone and especially the arid zone are those with seeds that could remain 
dormant until the most favorable conditions were present. Whereas the basically mesic zone, being less 
harsh, might allow a more haphazard "strategy" of immediate germination. Within this mesic zone are 
found the highest number of non-indigenous species studied, in fact, 66.7% of the species from this area 
are either introduced weeds or cultivated escapes, and the relationship between lack of seed dormancy and 
non-indigenous resident status has been mentioned in the previous section. 
Dormancy also appears correlated with dispersal mechanism. Of those species with seeds carried 
internally by birds, 88.9% show seed dormancy but only 16.6% of those carried externally by birds and 
36.3% of those introduced by man show dormancy. The one species with wind dispersal also shows 
dormancy. From these results it would appear that those seeds carried in the digestive tracts of birds need a 
protective covering, which could also act as a dormancy mechanism. Also suggested is the need of passage 
through the digestive system to break this dormancy, although in some cases destruction of the seeds has 
been known to occur after passage through birds (Rick and Bowman, 1961). In addition, all of the plant 
species studied here which are dispersed internally by birds have a widespread distribution throughout the 
archipelago indicating that this might indeed not only be a method of promoting germination, but also 
dispersal among the islands. The dormancy observed in Darwiniothamnus tenuifolius var. tenuifolius might 
also be correlated to its dispersal type since it is well known that aerial transport can at times be very harsh 
due to the extended time and low temperatures which might be encountered. Those seeds transported by 
man show the least amount of dormancy. This, again, can be explained by the fact that some weedy plants 
may be better suited if little or no dormancy is present. It is explained for the cultivated escapes by the 
simple fact that these plants are normally bred, either intentionally or not, so that dormancy is lost 
(Harper, 1965). Paspalum conjugatum appears to contradict this with its low germination. Perhaps it is best 
considered a native as Black (1973) suggests, rather than a cultivated escape. 
In summary, just over half, or 51.9% of the species tested show seed dormancy represented by less than 
25% germination. It appears from these tests that factors associated with this degree of dormancy are an 
indigenous resident status, an arid habitat, and internal dispersal by birds (other animals might also be 
important). However, dormancy is not an all or none situation in most cases. These tests do not preclude 
the possibility of a lesser degree of dormancy in the other species. Indeed, this would seem very likely since 
two characteristics of known value to many weeds and colonizers of open habitats are their ability to 
germinate in a wide variety of environmental situations, and continuous germination over an extended 
period of time (Mulligan, 1965; Ehrendorfer, 1965). The weedy character of the Galapagos flora and its 
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Noticias de Galapagos, vol. 44 1986
open pioneer habitat are well known (Porter, 1976, 1979; Hendrix, 1981), and many of these species may 
simply be demonstrating this "strategy" of staggered germination. This greatly increases the chances of 
sooner or later encountering suitable conditions for growth and perhaps establishment. 
ACKNOWLEDGEMENTS 
I would like to acknowledge with greatest appreciation the assistance received from Rev. and Mrs. c.A. 
McMullen in obtaining the majority of funding for this project. I also thank the Virginia Academy of 
Science for partial funding of the field work; and H.G. Bedell and J.L. Reveal for reviewing the 
manuscript. In addition, the following are acknowledged for their assistance: C. Shank, P. Zimmerli, 
G. Travez, J. Black, G. Reck, and the staffs of the CDRS and GNPS. 
LITERATURE CITED 
Black, J. (1973) Galapagos, Archipielago del Ecuador. Imprinta Europa, Quito. 
Carlquist, S. (1974) Island biology. Columbia University Press, New York and London. 660pp. 
Ehrendorfer, F. (1965) Dispersal mechanisms, genetic systems, and colonizing abilities in some flowering 
plant families. In: H.G. Baker and G.L. Stebbins (eds.), The genetics of colonizing species, 
pp. 331-352. Academic Press, New York. 
Harper, J.L. (1965) Establishment, aggression and cohabitation in weedy species. In: H.G. Baker and 
G.L. Stebbins (eds.), The genetics of colonizing species, pp. 243-265. Academic Press, New York. 
Hendrix, L.B. (1981) Post-eruption succession on Isla Fernandina, Galapagos. Madrono 28: 242-254. 
Johnson, M.P. and Raven, P.H. (1973) Species number and endemism: the Galapagos archipelago 
revisited. Science 179: 893-895. 
Mulligan, G.A. (1965) Recent colonization by herbaceous plants in Canada. In: H.G. Baker and G.L. 
Stebbins (eds.) The genetics of colonizing species, pp. 127-146. Academic Press, New York. 
Porter, D.M. (1976) Geography and dispersal of Galapagos Islands vascular plants. Nature 264: 745-746. 
Porter, D.M. (1979) Endemism and evolution in Galapagos Islands vascular plants. In: D. Bramwell (ed.), 
Plants and islands, pp. 225-256. Academic Press, London. 
Porter, D.M. (1983) Vascular plants of the Galapagos: origins and dispersal. In: R.I. Bowman et al. (eds.) 
Patterns of evolution in Galapagos organisms, pp. 33-96. Amer. Assoc. Adv. Sci., San Francisco. 
Rick, C.M. and Bowman, R.I. (1961) Galapagos tomatoes and tortoises. Evolution 15: 407-417. 
Ridley, H.N. (1930) The dispersal of plants throughout the world. Reeve, Ashford, Kent. 744pp. 
van der Werff, H. (1979) Conservation and vegetation of the Galapagos Islands. In: D. Bramwell (ed.) 
Plants and islands, pp. 391-404. Academic Press, London. 
Wiggins, I.L. and Porter, D.M. (1971) Flora of the Galapagos Islands, Stanford University Press, 
Stanford, Calif. 998pp. 
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Table 1. Plant data and results of tests showing the percentage of seeds germinating after exposure to Sodium 
Hypochlorite solution. The number of seeds tested is shown in parentheses. 
Plant Tested & Resident Dispersal 5 Minute 1 Hour Seed 
Study Site Status Mechanism Exposure Exposure Dormancy 
ACANTHACEAE 
Justicia galapagana (T) endemic birds (a) 67 (30) 0 (30) No 
ASTERACEAE 
Adenostemma platyphyllum (Z) introduced weed man 80 (40) 45 (40) No 
Ageratum conyzoides 
subsp. conyzoides (P) introduced weed man 61 (l00) 65 (100) No 
Bidens pi/osa (Z) introduced weed man 90 (50) 80 (50) No 
Darwiniothamnus tenuifolius 
subsp. tenuifolius (S) endemic wind 0(100) 0(100) Yes 
Jaegeria gracilis (P) endemic birds (a) 44 (50) 16 (50) No 
Scalesia pedunculata (S) endemic birds (a) 30 (50) 16 (50) No 
BORAGINACEAE 
Tournefortia pubescens (A) endemic birds (b) 33 (18) 44 (18) No 
Tournefortla rufo-sericea (T) endemic birds (b) 0 (50) 0 (50) Yes 
CACTACEAE 
Opuntla echios 
var. glgantea (A) endemic birds (b) 0 (50) 0 (50) Yes 
CYPERACEAE 
Cyperus elegans 
subsp. rubiglnosus (A) endemic birds (b or a) 24 (100) 27 (100) Yes 
EUPHORBIACEAE 
Croton scoulerl 
var. scouleri (A) endemic birds (b) 0 (9) Yes 
FABACEAE 
Cassia occidentalis (T) native birds (b) 10 (50) 2 (50) Yes 
LAMIACEAE 
HYPtis rhomboidea (P) introduced weed man 62 (100) 65 (l00) No 
LYTHRACEAE 
Cuphea racemosa (P) introduced weed man 86 (50) 66 (50) No 
MALVACEAE 
Bastardia viscosa (A) native birds (a) 4 (50) 6 (50) Yes 
Sida rhombifolia (S) introduced weed man 14 (50) 14 (50) Yes 
PLUMBAG INACEAE 
Plumbago scandens (T) native birds (a) 73 (11) 100 (9) No 
POACEAE 
Paspalum conjugatum (S) cultivated escape man 24 (50) 24 (50) Yes 
POLYGONACEAE 
Polygonum opelousanum (P) native birds (a or b) 48 (30) 39 (30) No 
PORTULACACEAE 
Portulaca oleracea (A) introduced weed man 77 (100) 74 (100) No 
RHAMNACEAE 
Scutia spicata 
var. paucif/ora (A) endemic birds (b) 0 (12) Yes 
SOLANACEAE 
Capsicum fruitescens (T) cultivated escape man 48 (25) 100 (25) No 
VERBENACEAE 
Clerodendrum moUe 
var. mo/le (A) native birds (b) 0 (20) 0 (20) Yes 
Lantana peduncularis 
var. peduncularis (A) endemic birds (b) 0 (30) 0 (30) Yes 
Verbena litoralis (Z) introduced weed man o (l00) 0(100) Yes 
ZYGOPHYLLACEAE 
Tribulus cistoides (A) introduced weed man 0 (34) 0 (34) Yes 
(A) arid zone, (P) pampa zone, (S) Scalesia zone, (T) transition zone, (Z) Zanthoxylum zone, (a) external, 
(b) internal 
Resident status and dispersal mechanism from Porter (1983) 
When two types of bird dispersal are indicated the first is considered most probable. 
24 
Noticias de Galapagos, vol. 44 1986


Seed germination studies of selected Galapagos Islands angiosperms

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Seed germination studies of selected Galapagos Islands angiosperms

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