244 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



it has the underground position of a root, and probably 

 performs some of the functions of a root. But it is 

 homologous with a stem — it is, in fact, an underground 

 stem, for jt has the structure of a stem, it bears leaves 

 like a stem, and in the axils of those leaves come buds, 

 making new shoots. Moreover, in many plants, as in 

 palms and in ferns, all gradations can be traced even 

 in the same family from the underground, through the 

 prostrate and inclined, to the upright position. 



(2) The so-called leaf of a cactus is no doubt analogous 

 to a leaf. It is flat and green, and performs the func- 

 tions of a leaf — viz., the assimilation of plant food. But 

 it is homologous with a stem. It is, in fact, a stem modi- 

 fied in shape and color to perform the function usually 

 performed by leaves. This is proved by its structure — 

 viz., pith, wood, and bark, with medullary rays connect- 

 ing the pith and bark ; and also by the gradations 

 among cactuses between the flat, leaflike form and the 

 cylindrical, stemlike form. Where, then, are the true 

 homologues of the leaves ? We find them in the spines. 

 These are abortive leaves modified as defensive organs. 

 They have the spiral arrangement of leaves, and in 

 their axils come the buds which form new shoots. 



3. One more example : The acacias — of which there 

 are in California about twenty species, introduced from 

 Australia — are by appearance easily divided into two 

 groups, viz., the feather-leaved acacias and the simple- 

 leaved acacias. These are so different in general appear- 

 ance, that the mere popular observer would probably 

 put them not only in different genera, but even in differ- 

 ent families. But doubtless the botanists are right in 

 putting them all in the same genus, for they are really 

 so closely allied that the same individual may pass from 

 one form to the other. 



The fact is, the so-called leaf of the simple-leaved 



