446 Ganong . — The Comparative Morphology of 
a clear green in all of these species, and large enough to do 
no inconsiderable amount of leaf-work, and wither with age 
instead of merging with the stem. They have good buds in 
their axils. Another feature which the embryos of these 
species have in common, and which may be correlated with 
their habitat, is the rapid growth of the hypocotyl and the 
relatively slower development of the cotyledons. In C. nycti- 
caiilis and triangularis the epicotyl forms first two leaves at 
right angles to the cotyledons, then two at right angles to 
these, thus forming four rows giving origin to four ribs. In 
C. nycticaulis one of these ribs soon splits into two, and the 
five then distribute themselves over the entire circle. This 
formation of new ribs by the forking or splitting of old ones 
is common in this family, indeed is the common method 
of forming the new ribs, and it shows how the rib once 
formed becomes, as it were, a true morphological element. In 
C. triangularis , on the other hand, one of the ribs soon stops 
abruptly; the one opposite to it keeps on its course, while 
the two intermediates turn spirally until they evenly occupy 
the circumference, after which the three persist through the 
life of the plant. In this early and temporary presence of 
four or five ribs in a characteristically three-ribbed species, 
we have another good case of repetition of phylogeny in 
ontogeny, a principle not very frequently illustrated by plants. 
In Cereus the morphological composition of the adults is 
extremely simple. The same axillary points produce first 
a cluster of spines and later a branch or flower. Hence 
nothing of special importance is to be expected in the seed- 
lings, for this is practically the kind of axillary bud which 
the seedlings produce throughout the family. 
4. Genus Phylloeactus. 
P. Ackermanni, Walp. Figure 18. 
P. anguliger , Lem Figure 19. 
P. phyllanthoides , Link, grandiflorus. Figure 20. 
P. Phyllanthus , Link. Figure 21. 
P. stenopetalus , Salm-Dyck. Figure in Goebel, 104 ; also Lubbock, II, 9. 
P. latifrons , Walp. Figure 6, PL II, in Goebel. 
P. Phyllanthus x C. fiagelliformis . Figure in Pfeiffer, XI, PI. XVI. 
