338 



BOTANY: PRINCIPLES AND PROBLEMS 



within the remains of the stout megaspore wall (Fig. 212). 

 On the exposed tissue, a group of archegonia appear. Each 

 microspore forms a single antheridium in which a group of 

 biciliate sperms develops. The young sporophyte is thrust 

 deeply into the tissues of the gametophyte until it has begun its 

 differentiation. Such a life history as this is clearly a step 

 in the direction of seed production. 



The genus Isoetes, the Quill wort (Fig. 213), is usually included 

 among the lycopods although its remarkable characteristics have 



Fig. 212. — Female gametophyte of Selaginella. The stout wall of the micro- 

 spore still encloses part of the gametophyte. At the right is an archegonium; at 

 the center and left, young embryos which have arisen from fertilized eggs in 

 other archegonia. The embryo is carried down into the tissue of the gameto- 

 phyte by an elongated cell, the suspensor. In the larger embryo, the shoot (at 

 right) and root (at left) are beginning to become differentiated, as well as the 

 large, absorbing foot, at the lower left. (Mainly after Pfeffer). 



caused some botanists to place it in a distinct order. The plants 

 grow in water or very moist places and each consists of a tuft of 

 long, quill-like leaves, arising from a short and flattened stem. 

 In the hollow bases of these leaves the sporangia are borne. 

 Isoetes is heterosporous, its gametophytes being similar in general 

 structure to those of Selaginella except that the sperms have 

 many cilia. 



The lycopods were particularly prominent in the forests of the 

 Coal Period, the great tree-like lepidodendrids and sigillarians 

 belonging to this order. It is noteworthy that these ancient 

 members of the group possessed a cambium and well developed 

 secondary wood, tissues which are quite absent in living lycopods. 



