162 COMPENDIUM OF GENERAL BOTANY. 



their appearance later. This fact, however, has no bearing on the 

 theory of descent (evolution), as might be supposed.' There is 

 no doubt that our most highly organized plants started from a 

 single cell ; of this ontogeny has given abundant ])roof. But to 

 conclude from this that all plants in their successive generations are 

 evolved lyhylogenetically is strange speculation. From arguments 

 founded upon a natural basis we cannot accept such a hypothesis. 



The most important facts in regard to the physiology of cotyle- 

 dons have already been mentioned under assimilation. 



2. Cataphyllary leaves. These leaves occur below the foliage- 

 leaves; they not only occur near the base of the stem, but may be 

 found near the base of branches. As already indicated, they are 

 scaly and the blade-portion of the leaf predominates. The bud- 

 scales, which serve to protect the bud during the winter months, 

 are usually such cataphyllary leaves. As the name indicates, they 

 are situated at the base of the future stem or branch ; they, of 

 course, are situated at the apex of the stem during the summer and 

 autumn, that is, above the foliage-leaves of the older generation. 

 The foliar structures of the above-mentioned subterranean stem- 

 organs are cataphyllary leaves; for this reason tlie rhizome, the 

 bulb, and the tuber are sometimes called " cataphyllary leaf-stems." 



3. Foliiige-l eaves. The green leaves, usually recogrn'zed as 

 leaves, are the typical organs of assimilation. Nearly all that has 

 been stated in regard to the special n)oiphology and physiology of 

 leaves had reference to the typical assimilating leaves. Movements 

 to place them in suitable positions with regard to sunlight, etc., 

 will be discussed in a subsequent chapter. 



4. HypsophyUary leaves. The hypsophyllary leaves, also called 



' Nor can we accept Hackel's "biogenetic law," which states that phylogeny 

 is repeated in ontogeny. For example, the embryo of the ferns (Cerntopteris) 

 leaves its "thallorae" state verj^ early and forms the beginnings of a stem, root, 

 and leaves, although " phylogeneticallj'^ " it is certainly more closely related to 

 the thalloid plants than to the phanerogamic embryos. If its thalloid nature is 

 prominent in the idioplasm, why does it leave its thallome state earlier than does 

 tho embryo of phanerogams ? Moreover, it follows (in opposition to Nageli) 

 from the conceptions of stem, leaf, and thallome that a differentiation into stem 

 and leaf must be preceded b\^ a tlialloid state, since leaf and stem are correliited 

 terms. Nothing else aeeiw?. possible Waxn that a thalloid structure of one or more 

 cells must precede the formation of stem and leaf. (Though manj^ problems 

 connected with the theory of descent are still unsolved, yet, in general, it is unde- 

 niable that the pliylogenetic history of the individual, is so to speak, reflected in 

 the ontogenetic development.— Tkans.) 



