THE SPERMATOPHYTA : GENERAL INTRODUCTION 651 



maintain that they are merely superficial outgrowths. The evidence is 

 inconclusive and the question must await future solution. 



Zimmermann bases his ideas upon simple vascular plants, but we may 

 quite reasonably suppose the sort of differentiation he depicts to have come 

 about at a much earlier stage of evolution, during the development of branch- 

 ing in the primitive algal filament, and to have been carried over directly into 

 the simple vascular body as cortication advanced and the axes grew more 

 massive. Telomes may represent the descendants of the earliest ramuli of 

 the chlorophycean thallus, more massive indeed, but morphologically 

 homologous. 



Zimmermann 's theory, valuable as it is, nevertheless, stops short of 



A B C 



Fig. 657. — Diagram showing three ways in which axillary branching may have been 

 achieved. The portions of the axis which are marked in outline in each case are 

 assumed to have been suppressed. A, i, Fertile and sterile telomes separated by 

 internodes. A, 2, Alternate internodes suppressed. B, i, Fertile and sterile 

 telomes on lateral mesomes. B, 2, Basal internode of lateral mesomes suppressed. 

 C, I, Fertile and sterile telomes in dichotomous pairs. C, 2, Basal internodes of 

 lateral mesomes suppressed. (After Zininiermann.) 



accounting for the origin of the branches or leafy shoots of a higher order, 

 such as are commonly found in all vascular plants even in the simplest 

 Lycopsida. It is clear that all the segments of the original axial system cannot 

 have been reduced, as he suggests, to phylloids or sporangia respectively, 

 both of which are structures of limited growth. Some must have retained 

 the capacity for unlimited apical growth and for the production of fresh 

 telomes. Such a differentiation must have happened, though we can only 

 guess at its mode. The " overtopping " process which established the main 

 axis, would presumably also apply to the branching of the lesser segments 

 or laterals. In their branch systems also one axis would remain predominant, 

 retaining its radial symmetry and apical growth, while the others (telomes of 

 the third order) would be reduced to the specialized appendages of determinate 

 growth, leaves and sporangia. 



Whether the still smaller appendages of the plant body, the hairs, spines, 

 glands, etc., which are collectively known as trichomes, are derived from 

 very reduced telomes is decidedly doubtful. Many of them originate as direct 

 outgrowths from single surface cells, and there is no line which can be drawn 

 to separate such simple outgrowths from more highly developed trichomes. 



