434 Ganong. — The Comparative Morphology of 
subject will be in a measure a supplement to the present 
one. I need only state that the chief characteristics of the 
cellular anatomy of the germinated embryos through the 
family are as follows : — epidermis, one layer of cells, thin, 
often papillate ; cortex of very large round cells, sometimes 
visible to the naked eye, and forming the greater part of the 
hypocotyl, from four to six times the diameter of the central 
cylinder (Figs. 2 d, 5 d ) ; central cylinder very simple, of two to 
six fibro-vascular bundles. The young root is characterized 
by a great development of cork, slight development of cortex, 
and a condensation of the central cylinder. Naturally this 
simple structure becomes much modified in the seedlings, and 
still more in the adults. 
The Colour-Factors of the Embryos. 
The germinated embryos in this family show a considerable 
range of colours, from a clear typical chlorophyll-green in some 
Platopuntiae and climbing Cerei, to a deep red in Mamillariae 
and others. As the embryos lie in the seeds they show no 
colour in any of the species I have observed, except the usual 
translucent white. On emerging from the seed, they turn 
at first pale green, which deepens upon some but in others 
is soon masked by the red, or, as in Cylindropuntiae, becomes 
olive or greyish green. In Echinocactus viridescens , for 
example, as it breaks out from the seed, the embryo turns 
at first pale green, but very soon a blush of clear red appears 
on the tips of the cotyledons, and spreads over all the upper 
part of the hypocotyl, and it appears vividly also on the first 
two leaves of the epicotyl. One of the most brilliantly red 
species I have noticed is Mamillaria Nuttallii. Here and 
there over these red embryos, one sees brighter greenish 
spots, which are areas under the stomata. Examination 
shows that the red colour is in the sap of the epidermal cells. 
Moreover its appearance is dependent upon light, for not 
only does it appear only on parts exposed directly to the 
light, even to the extent of forming only upon the upper 
side of a hypocotyl when this is lying upon its side, but in 
