464 Ganong . — The Comparative Morphology of 
I have not yet been able to determine the important point of 
the mode of formation of the inner vegetative point, but have 
no doubt it comes about as in Thelocacti and Mamillaria. 
It should be particularly plain in A. fissuratum . 
A feature which is prominent in this genus and not found 
in any other Cactaceae, so far as I know, is the early growth 
in succulence of the root along with the hypocotyl. The 
root, shortly after the embryo is out of the seed, instead 
of remaining small, begins to swell not only as fast as, but 
often faster than, the hypocotyl. For a time a constriction 
marks the boundary between the two (shown in Fig. 50 c), 
but later this disappears, and in the adult plant the true 
epicotyledonary stem is extremely short, while the great 
visible part below the tubercles is really root, as Goebel 
has already surmised. This unusual succulence of the root 
is no doubt correlated with the half-buried habit of the plant. 
Succulent roots of another kind occur also in Cereus tuberosus , 
Echinocactus napinus , and perhaps other species. 
1 8. Genus Pelecyphora. 
This genus, with its single odd species, is but slightly 
removed from Mamillaria , with which it is no doubt con- 
nected through a form like M. micromeris , which somewhat 
closely approaches it in many characters. From my seeds 
several embryos were produced which, however, did not differ 
from those of the smaller kinds of Mamillaria. The coty- 
ledons were very minute on a nearly globular hypocotyl. 
Summary of the Form-factors of Embryos and 
Seedlings. 
We may now group together the facts just described and 
note to what conclusions they point. Taking first the 
germinated embryos, it is plain that as a whole there is, from 
Pereskia through Cereus and Echinocactus to Mamillaria , 
a progressive condensation in bulk, and diminution of surface 
