656 A TEXTBOOK OF THEORETICAL BOTANY 



as some do, condemn it outright. The mind cannot be restrained from 

 attempting to build for itself a coherent working hypothesis of the unknown, 

 and such imaginative constructions, if they are not allowed to degenerate 

 into dogmas, may sometimes be serviceable guides to investigation. 



Summary of Theories regarding the Spermatophyte Shoot 



The discussion of the ideas of theoretical morphology which we have 

 attempted in this chapter is necessarily involved and may be difficult at first 

 to follow. It may be useful, therefore, to sum up the main points. 



1. The shoot may be interpreted as formed of two distinct categories 



of organs ; stems and leaves. The latter are regarded under the 

 Strohilar Theory as all primitively fertile but subject to progressive 

 sterilization, which eventually hmits the fertile zones to the ends 

 of the branches. 



2. The shoot may be alternatively regarded under the Telome Theory 



as primitivelv undifferentiated, composed of leafless axes, the 

 mesomes, bearing terminal segments, the telomes, which may be 

 either sterile or fertile. The body was originally dichotomous, 

 but by a process of overtopping may have become monaxial. 



-x. According to this second view leaves and sporangia are alike axial 

 structures, both being evolved from the primitive telomes. The 

 axillary position of the sporangia has been arrived at secondarily. 



4. While microphyllous leaves, such as those of the Lycopodiales, are 

 regarded as derived from single telomes and are called phylloids, 

 the megaphyllous leaves of Ferns and higher plants are considered 

 as syntelomes, that is, compound structures equivalent to an 

 entire shoot of a microphyllous plant. They are called phyllomes. 



c. These phyllomes are believed to have become integrated in the 

 Spermatophyta into sympodia which have replaced the original 

 axis by a compound structure. The various views as to the nature 

 of this compound axis are called phytonic theories. According 

 to the interpretation here advanced, such views are only applicable, 

 if at all, to the Spermatophyta, while in all the Vascular Cryptogams, 

 except possibly the Marattiaceae, the axis is regarded as represent- 

 ing the primitive mesomes of the earliest land plants, that is to 

 sav, structures which may be ultimately traceable to the cellular 

 filaments of primitive Thallophyta. 



