164 de Fraine.—The Seedling Structure of certain Cactaceae. 
moving laterally, until the opposite groups finally fuse, and a diarch root 
results. The transition is thus seen to be very similar to that illustrated 
in Diagram 14, Figs. 3-6, but the four groups are in M. pusitla all epicotylar 
in origin. 
Value of Seedling Anatomy in Classification. 
The Cactaceous seedlings examined can be classified into two groups so 
far as their external morphology is concerned. The first of these groups 
includes those seedlings which bear a more or less close resemblance to those 
of an ordinary Dicotyledon, having a long hypocotyl and distinctly leafy 
cotyledons. Three genera are included in this division, namely, Pereskia, 
Opuntia and Nopalea ; the seedlings of the first show no sign of a succulent 
habit, but in the last two genera evidence of it is given in the somewhat 
swollen hypocotyl and in the thick seed-leaves. 
The second group is composed of all the other genera which have been 
examined, viz. Phyllocactus , Cereus, Pilocereus , R hip satis, Echinocereus, 
Echinopsis , Echinocactus and Mamillaria. The members of this group show 
a considerable diminution in length and a marked increase in succulence as 
compared with the seedlings of the first group ; in them the hypocotyl is 
always short and is ovoid or globular in shape, while the cotyledons are 
small, pointed structures often microscopic in size. 
Generally speaking, two types of transition are found in the order; 
the Anemarrhena type of Miss Sargant is characteristic of the seedlings 
of Group I, while Van Tieghem’s Type 3 forms the ground-plan of the 
transition phenomena of the members of Group II ; in this respect the 
seedling anatomy follows the morphology. 
Group I. 
In the seedlings of this group the transition in the upper part of the 
hypocotyl follows closely the Anemarrhena type, with the exception of 
Opuntia maculacantha and O. Pseudo-tuna. The type is maintained and 
a tetrarch root is established in the usual way in Nopalea n. sp., Opuntia 
Ficus-indica, O. Tuna, O. polyantha , O. imbricata , O. Bergeriana, and 
O. stricta Series A and C, but a modification of it occurs in O. stricta 
Series B, O. albicans, Pereskia n. sp., and P. Pititache. The seedling 
structure is, however, of very little use in delimiting the genera in the group, 
for the resemblance between two species of the same genus is sometimes 
much less close than between species of different genera; thus Nopalea n. sp. 
and Opuntia Ficus-indica , or Opuntia albicans and Pereskia n. sp., would 
appear from their seedling anatomy to be much more closely related than 
Opuntia Ficus-indica and 0 . albicans. 
