392 
PACIFIC SCIENCE, Vol. XVIII, October 1964 
TABLE 1 
Variation in Leaf Shape in 11 Populations of Gossypium tomentosum 
COLLECTED FROM FOUR DIFFERENT ISLANDS* 
( Measurements based on ten mature leaves per population ) 
L — Length of leaf in mm from pulvinus to tip of median lobe 
S — Length of sinus in mm from pulvinus to base of median lobe 
Index — S expressed as a percentage of L 
L 
5 
Index 
Mean 
Range 
Oahu 
1. 
Barber’s Point — Kahe Point 
84.1 
32.2 
38.3 
31-49 
2. 
Kahe Point — Nanakuli 
78.8 
30.8 
39.1 
28-49 
3. 
Maile Point (procumbent form) 
80.0 
29.0 
36.3 
30-45 
4. 
Maile Point (shade form) 
100.0 
39.4 
39.4 
31-46 
5. 
Makapuu Point 
79.6 
32.4 
40.7 
36-47 
Molokai 
6. 
Kaunakakai 
74.7 
28.8 
38.6 
27-50 
7. 
Onini Gulch (100 ft elev.) 
64.4 
24.7 
38.4 
35-45 
8. 
Onini Gulch (200 ft elev.) 
81.0 
34.6 
42.7 
36-49 
Lanai 
9. 
Keomuku 
71.1 
32.4 
45.6 
38-53 
10. 
Manele 
71.7 
31.2 
43.5 
36-55 
Maui 
11. 
Pohakea Gulch 
71.1 
33.0 
46.4 
42-51 
tions in both characters appear to be essentially 
random. On the other hand, variations in pubes- 
cence and plant habit were markedly correlated 
with local environmental conditions to which 
the plants happened to be exposed. Plants grow- 
ing in exposed situations usually had a sprawling 
or semiprocumbent habit, and the leaves were 
covered with a dense tomentum giving them a 
bluish-grey appearance. Under Prosopis cover 
individual plants were found which grew as high 
as 9 ft, and the leaves were finely tomentose 
with a dusty green color. 
Experimental studies have shown that in New 
World cottons, pubescence and plant habit can 
be influenced strongly by environmental con- 
ditions, particularly light intensity; both charac- 
ters are also sensitive to changes in genotypic 
background. For instance, a single major gene 
determines the finely tomentose surface of to- 
mentosum. When the gene is transferred by 
backcrossing to a hirsutum background, the leaf 
surface becomes densely hairy and phenotypi- 
cally indistinguishable from the hirsutum mutant 
form "Pilose.” The degree of hairiness can also 
be modified to a lesser extent by transferring the 
plants from a shaded greenhouse to full sunlight. 
Comparative studies under controlled environ- 
mental conditions would therefore be necessary 
to determine whether the variation observed in 
natural stands of tomentosum is phenotypic only 
or the result of ecotypic differentiation. 
POLLINATION AND INTROGRESSION 
1. The Primary Breeding System 
It has been pointed out by Baker ( 1955 : 347— 
349) and Stebbins (1957:343-344) that an 
autogamous breeding system would be initially 
advantageous to a colonizing plant species, since 
it would render the latter independent of pollen 
vectors. On the contrary, it would be most un- 
likely for appropriate pollen vectors to be in- 
troduced into a new habitat along with the 
cross-pollinated species dependent upon them. 
This argument is particularly pertinent if ap- 
plied to the colonization of isolated volcanic 
islands like the Hawaiian group. 
All species of Gossypium are self-compatible 
with radially symmetrical flowers, large numbers 
of anthers, and a true floral nectary located in a 
deep circular groove lined with glandular hairs 
at the inner base of the calyx. The nectary can 
be seen only under low-power magnification of 
a vertical section of the receptacle. It is accessible 
both to long-tongued insects and to insects small 
enough to crawl between the bases of the petals. 
Usually there are also extrafloral nectaries lo- 
cated ( 1 ) at the bases of the bracteoles and ( 2 ) 
