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A TEXTBOOK OF THEORETICAL BOTANY 



The third viezv involves an attempt to harmonize the thalloid type of 

 organization with the radial type characteristic of vascular plants, for which 

 there is no warrant. It is true that so far as the Bryophyta are concerned the 



Fig. 653. — Diagram illustrating the process of " overtopping " to 

 produce a monopodial shoot from a dichotomous shoot. 



establishment of an axis with leafy appendages from a flat dichotomous thallus 

 may possibly have occurred, and Fig. 654 shows an example of existing 



forms which, by comparison, illustrate how it may 

 have come about. There is, however, no evidence 

 that the vascular plants had thalloid, dorsiventral 

 ancestors, and the probabilities are against it. 

 Among the Algae radially organized and thalloid 

 types are both frequent and are quite independent. 

 The difference of habit, based upon the filament 

 and the flat plate of cells respectively, may be 

 traced even amongst the simplest forms. There 

 is thus no reason for excluding the possibility 

 that the radial type of organization might be 

 ultimately derived from the filament form, by 

 direct descent, without passing through a thalloid 

 phase. This third view is, in fact, an illegitimate 

 extension to primitive conditions of the phytonic 

 ideas of construction, considered later in this 

 chapter, which are derived from the higher plants 

 and should be applied only to them, if at all. 



Relationships of Stem and Leaf 



We shall obtain a clearer insight into these 



Fig. 654. — Aneura eriocauUs. 

 Complete plant to show the 

 differentiation of a thallus ,, .^ -ji-n ■ ^ c 



into an axis with leafv problems it wc Consider briefly some points or 



appendages and leafless structure in existing leaves and try to judge 



branches at base function- 111 n 11 -^i 



ing as roots. {After Goebel.) whether they are all comparable organs with a 



common evolutionary origin. 

 Jeffrey has emphasized that in the living groups Lycopodiales, Equisetales, 

 Psilotales and probably the Isoetales, the leaves are small in relation to the 

 axis and that they are supplied by vascular strands which cause no gap where 



