the Embryos and Seedlings of the Cactaceae. 467 
I think the seed does influence the form of the embryo, as for 
example in the difference of size between the two cotyledons 
in many species, particularly Platopuntiae. Observation 
shows that the smaller is on the concave side, covered by the 
convex larger one. Again, possibly the forking of the tip of 
one cotyledon in Cereus Bonplandi may be due to this, and 
certainly the one-sidedness of the root in E. myriostigma and 
E . capricornis , and the slight asymmetry in both planes 
noticed in nearly all cotyledons and hypocotyls, including the 
slight pointing to one side of the pointed cotyledons in Cereus 
and Echinocctctus . But such are very minor effects. 
Of course in any given generation the form of the embryo, 
aside from slight irritable responses to light, &c., is determined 
by heredity. But heredity is but the sum and resultant 
of past experiences, and hence in the present case is largely 
a study of the facts of past environments. This suggests an 
explanation which I believe to be the true one, i. e. that the 
form of the adults, like any other character, once acquired — it 
matters not for our present purpose how — as it becomes more 
and more fixed and intensified, tends to work back into earlier 
and earlier stages in the ontogeny of the successive indi- 
viduals ; until finally, a character adaptively acquired by 
the adults works back into the epicotyl, of which I have 
shown many cases in this paper, and finally into the embryo 
itself. But while the working back into the epicotyl may be 
comparatively rapid, the passage into the embryo seems to 
be very slow, as is explained by the considerable lagging of 
those behind the adults, due no doubt to the fact that the 
embryos have a set of activities of their own in their early 
life which keeps them from being too plastic to other influences 
working upon them. 
In summary then the shapes of the embryos in Cactaceae 
seem to be determined by three leading factors. First, the 
ground-form of the embryos answers in general to the ground- 
form of the adults, and alters with the latter by the working 
back of newly acquired characters. Second, there is an 
occasional expansion of surface, with increased cotyledons 
