(ol 
Vol. VI., No. 31.] BULLETIN OF THE TORREY Boranicat Cuus. [New York, July, 1877. 
§ 163. Separation of sexes in trees.—Mr. Darwin has called 
attention to the fact that trees have a strong tendency to bear flowers 
with separated sexes (Orig. Spec., Chap. 1V., Cross and Self Fert., 
Chap. X.). He finds that about 60 per cent. of the native trees and 
bushes of Great Britain are diclinous, “an enormous proportion com- 
pared with other British plants.” From Dr. Hooker it appears that 
in New Zealand 48 per cent. of the trees and 41 per cent. of the 
bushes have this character, but only 264 per cent. of other plants. 
Dr. Gray’s authority is given for the statement that out of 132 trees, 
native to the United States, 95 or 72 per cent. are more or less di- 
clinous, for the most part decidedly so. As Dr. Gray has evidently 
limited himself to the taller trees, the 419 “ trees which usually attain a 
height of 16 feet or more” in Dr. Vasey’s catalogue were considered 
from this point of view. It was estimated that 268 or 64 per cent. 
were more or less diclinous. This result compared with that of Dr. © 
Gray and that of Dr. Hooker shows that the bushes diminish the 
percentage, or in other words that the higher the trees the more 
likely they are to be diclinous. In Gray’s Manual there are enumer- 
ated 2562 flowering plants; of these 493 or 19} per cent. may be 
said to be diclinous. Of trees, shrubs, and woody-climbers which 
usually attain the height of at least six feet there are 292, and 169 
~ of these, or about 58 per cent., are diclinous. If there were no ten- 
dency of woody-plants this way there shouldbe only 194 per cent., 
or s6 instead of 169 diclinous. As in this calculation there are 
included bushes only six feet high, the lower percentage, 58, seems 
to confirm the rule, that the higher the tree the greater the proba- 
bility of its sexes being separated. 
But it is only a rule subject to exceptions and notalaw. The 
Australian Gum-tree, said to be the tallest of trees, does not appear 
to be diclinous, and in fact Dr. Hooker says the rule does not hold 
good in Australia at all. Nor does the converse of the proposition, 
the more diclinous plants the more trees, hold in. reference to the 
Endogens in Gray’s Manual. There are found (in that class) 711 
species of the 2562 flowering plants. A simple proportion gives 81 
for the number of trees to be expected among Endogens out of the 
total of 292. But as 205 of the 493 diclinous plants are Endogens, 
that is about 414 per cent., and there are 169 diclinous trees in all, 
about 70 would be the very least number to be expected. In fact 
there are not 81 nor 70 but only 8 endogenous trees, all diclinous, it 
is true, but such insignificant representatives of trees as the 8 woody 
Smilaces. 
It seems to be now pretty well settled that the pollen is conveyed 
from the anther to the stigma chiefly through two agencies, the wind 
and insects. This must be especially the case with diclinous plants. 
If the flowers are to attract insects, they must do so by putting up a 
sign that they have honey and:pollen to dispose of. The signs which 
insects read are color and odor. If, on the other hand, fertilization 
depends upon the wind, there must be an over supply of pollen, and 
a crowd of recipients with prominent stigmas to secure the continu- 
