THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



633 



The stem segment then cuts off two more cells which give rise to the second 

 and third leaves, thus forming the first nodal whorl of the young plant. 

 Meanwhile the lower cell has divided up to give rise to the main root and 

 the foot, the latter being a comparatively unimportant structure in Equisetum. 

 Very early in development the apical cells of the stem and primary root are 

 organized (Fig. 646). 



Growth now continues rapidly and the root bursts through the tissues 

 of the prothallus, whilst the young stem grows out through the neck of the 

 archegonium, which forms the calyptra, and the young plant becomes 

 independent, though for a time it retains connection with the prothallus 

 through the foot. In course of development the stem becomes organized in 

 the form of the mature plant, whilst the root gradually disappears, and, as 

 we have seen, its function is taken over by adventitious roots which arise 

 from the underground part of the developing stem. 



The primary stem grows upright until it has formed about a dozen nodes, 

 each of which bears three leaf-teeth. It is then replaced by a more vigorous 

 branch arising near the base of the first stem, and this in its turn by another 

 in succession. The third or fourth of this series of branches turns down- 

 wards into the earth and becomes the rhizome of the mature plant. 

 Anatomically the primary stem is peculiar in being solid, without the carinal 

 canals which are present in the adult stem, and the xylem is better developed 

 than in later shoots. At the base of the seedling there is a protostele, but 

 the axis becomes siphonostelic at the level of the first branch. 



Alternation of Generations 



We see in the life-history oi Equisetum the same alternation of generations 

 we have met with in the other examples of the Pteridophyta. There are, 

 how^ever, certain interesting points w-hich arise in this example. Unlike 



Equisetum Plant 



Cone 



Sporangium 



Spores 



Antherozoidi 



Prothallus ■ 

 Fig. 643. — Life-cycle oi Equisetum arveme. 



21 A 



