the Embryos and Seedlings of the Cactaceae. 437 
or prevailing size of adults in the genus and that of the 
seeds and embryos. Thus Pereskia , including great shrubby 
climbers, has the largest embryos ; next, Opuntias have the 
largest adults, seeds, and embryos ; Cereus and Echinocactus 
are in all three respects much smaller, the latter more so than 
the former ; Mamillaria and Anhalonium are in all respects 
smallest of all. Considering the natural relationships of these 
groups, we may say that the seeds in the Cactaceae have 
phylogenetically grown progressively smaller, just as the adult 
plants have done, taking the embryos with them. In general, 
though with some exceptions, this progressive diminution in 
size of the adults accompanies an increasing dryness of habitat. 
It is then probably true that the size of the embryos is 
reduced, not directly by the dryness, but that it accompanies 
the reduced size of the adults and seeds ; for if the former 
were true we should find much greater variation in size of the 
embryos than we do. A marked and important exception to 
the reduction in size is found in the climbing Cerei and 
Phyllocactus , where the embryos are much larger than those 
of the desert Cerei, and have leaf-like cotyledons. But the 
forms possessing these larger embryos have abandoned the 
desert for a life in the woods, where the mesophytic conditions 
allow of a much larger spread of surface, and in this case it is 
plain that the embryos themselves respond to these conditions 
and increase in size, a point which will be discussed later. 
Another and important incidental feature of size in the 
embryo is its increase when the epicotyl is not allowed to 
develop. In my plants, sometimes by accident and some- 
times by design, the epicotyl and axillary buds of the 
cotyledons became removed ; in such a case no new buds 
formed, but the embryo continued to increase in size until 
it became double that of the normal (Fig. 2 e). What this 
must mean is, that a certain amount of food-substance which 
would normally have gone into the building of the epicotyl, 
is here diverted into making larger the cells of the embryo 
(cotyledons and hypocotyl). This growth of the embryo 
under such conditions would be difficult to explain on the 
