THE CANONS OF COMPARATIVE ANATOMY 235 



lation. It is assumed in connection with this doctrine that the 

 young of any species may hi the course of its individual development 

 pass through the phases present in ancestral forms. As examples 

 of this principle we may take extreme types of vegetation, such as 

 the almost leafless cacti, forms with phylloclads, or those character- 

 ized by the presence of short-shoots. In the mass of Cactaceae the 

 leaves are abortive and are represented at most by spines. In the 

 seedlings, in contrast to the adult, 

 the foliar organs are distinctly pres- 

 ent and are clearly recognizable as 

 such. In coniferous species belong- 

 ing to the genus Phyllodadus the 

 branches in the mature state form 

 flattened expansions known as 

 phylloclads. If a seedling of any 

 species of Phyllodadus be exam- 

 ined, it becomes clear that a nor- 

 mal round and leafy axis is present 

 such as ordinarily characterizes the 

 conifer (Fig. 175). Likewise hi 

 the pine the seedling shows the 

 primary leaves arranged on the FlG ' 175. -Seedlings and mature 



. i r i . r branch of Phyllodadus species. 



stem in the usual fashion for 



coniferous gymnosperms and not clustered on short-shoots or 

 brachyblasts as in the adult branches of the genus. Further, 

 hi a conifer like the larch, which is differentiated in habit from the 

 mass of the group by its deciduous foliage, we find in the seedling 

 that the leaves persist for several years, thus revealing the probable 

 ancestral condition for the genus. An additional example among 

 the dicotyledons is supplied by the oak. The adult in north- 

 ern oaks is characterized by deciduous leaves. Oak seedlings and 

 saplings, however, even in the case of typically northern species, 

 retain their leaves during the winter, thus recalling a situation 

 characteristic of the live oaks of warmer latitudes which have 

 evergreen foliage and represent anatomically the primitive type 

 of organization. 



