FLORIN: SYSTEMATICS OF THE GYMNOSPERMS 349 



2. When species represent an evolved stage of the phylum, their juvenile forms are 

 simply intermediate between the embryos and the adult forms; these species are evolved 

 both in the juvenile and the adult phase. 



3. When a species represents an overevolved stage of the phylum, its juvenile forms 

 are evolved, and its adult forms overevolved; here the adult form is more evolved than 

 the juvenile, however, and presents, before the juvenile form, indications of a return 

 towards the ancestral form. Senile forms sometimes appear at the end of the evolution 

 of a phylum. 



The type of evolution characterized by an apparent return to primitive con- 

 ditions is called pseudocyclic (Gaussen, 1952). 



The nature of the alternation of generations remained a matter of contro- 

 versy. Bower (1929, 1935) retained the same general attitude as in 1908, al- 

 though modifying it in the light of later work. He thus restated his antithetic 

 theory, now called the theory of interpolation, and contrasted it with the homo- 

 logous or transformation theory. Zimmermann (1949) emphasized that the 

 arguments against Bower's theory did not disprove the view that the divergent 

 differentiation of the two alternating generations is an adaptation to life on 

 land. In later years problems of this kind have been discussed mainly from the 

 genetic point of view. 



The organization of the sporophyte body of vascular plants in general, and 

 the "telome" theory (Zimmermann, 1930, 1949, 1952; cf. Ilalle, 1933; Bower, 

 1935; Eames, 1936; Lamm, 1948, 1952; Stebbins, 1950; Florin, 1951) in particu- 

 lar, attracted great attention. This is a phylogenetic theory proceeding from the 

 structure of known primordial plants, and combining results of research on 

 external form, internal structure (stelar theory), and ontogenetic development 

 (alternation of generations). Its main points are as follows: 



1. The vascular land plants originated from seaweeds with dichotomously branching 

 thallus. Primitive telomes ("Urtelome") derive from unicellular stages by combination 

 of cells, formation of meristem (origin of polarity), rotation of cell axis, shifting of main 

 phases in alternation of generations, and formation of various permanent tissues. 



2. The first vascular plants were composed of undifferentiated uniform organs, or 

 telomes in a wide sense. Such telomes comprise telomes in a restricted sense — the ulti- 

 mate uninerved segments of a dichotomizing branch system — and mesomes, which are 

 similar segments between subsequent points of forking. Both were protostelic. The 

 telomes were divided into vegetative telomes or phylloids, and fertile telomes or terminal 

 sporangia, producing spores. 



3. The evolution of the vascular plants in all subsequent geological periods is the 

 result of a few basic morphological trends: overtopping, planation, syngenesis or fusion, 

 reduction, recurvation, and longitudinal differentiation. 



The shoot of seed-plants was studied by Bower (1930) from the point of 

 view of the relation between size and form. Various aspects of the relationships 

 between stem and leaf have been subjected to study. Arber (1950) regarded the 

 leaf as a partial shoot borne by a whole shoot, and with an urge towards self- 

 completion as a whole shoot. The phytonic theory of the phyllorhize, involving 

 a root attendant on each leaf, was discussed by Boureau (1939, 1952) on the 

 basis of ontogenetic investigations of the anatomy of seedlings in the Pinaceae 

 and other conifers. Bower (1935), Eames (1936), and Emberger (1952a), how- 

 ever, considered the phj^tonic theories valueless. The evidence of the organiza- 

 tion of primary shoots was characterized by Wetmore (1943) as still too in- 

 complete to permit of generalizations. Barthelmess (1935) and Esau (1943) 



