BOTANICAL SUBJECTS. 



121 



This primary frond emits numerous similar leaves which issue 

 irregularly from the thick midrib, and in their turn emit secondary, 

 and often tertiary similar, but smaller leaflets." 



It is evident from this that there is no distinction between 

 stem, petiole, midrib, veins and veinlets. They are smaller and 

 smaller sub-divisions of the stem. Indeed, this is what Le Maout 

 and Decaisne say (" Syst. of Bot.," p. 130). 



*' The relative position of the fibro-vascular bundle, which 

 passes from the stem into the leaf, shows clearly that the leaf- 

 blade may be compared to a flattened stem, the fibres and vessels 

 of which have been spread out, and thus allowed plenty of room 

 for the development of parenchyma between their ramifications." 



In the case of this Delesseria, the stem was first a midrib, 

 and then after being divested of its lamina by decay, it hecame a 

 stem. 



At first there was no such things as fibro-vascular bundles. 

 The stem, midrib, and laminae were simply cellular, as in 

 Delesseria, and the network of their cellular interspaces was their 

 vascular system — an adumbration of the more differentiated 

 vascular system of the higher plants of to-day. Fig. 13 gives an 



Fig. 13. Cross section of frond (magnified) of Rytiphlcea simplicifolia, 

 Han-. ("Harvey's Phyc. Austr.," pi. 245).* 



idea of how the fibro-vascular system of phtenogams may have 

 developed out of the spaces left between the cells of cryptogdms . 

 These had no other circulatory channels but the spaces between 

 their cells. Indeed, they had not much need of a more elaborate 

 system of circulation for not being encumbered bv a so-called 



* In Polyphacujn proUferum, Ag. pi. 188, the same vascular-like inter- 

 cellular spaces are seen. 



