226 



PLANT MORPHOLOGY 



initial divides by a periclinal wall to form an outer and an inner row, the 

 former constituting the primary wall cells and the latter the primary 

 sporogenous cells (Fig. \8W, D). The sporangium wall becomes at least 

 three layers of cells thick, the inner layer forming the tapetum (Fig. 



\^\E-G). This is a nutritive layer 

 that, instead of disorganizing, as in 

 the horsetails and ferns, remains 

 intact for a long time. After the 

 sporogenous cells have increased 

 in number, spore mother cells are 

 organized and from each a tetrad of 

 spores arises (Fig. 181//). The 

 development of the sporangium of 

 Lycopodium takes place according 

 to the eusporangiate method. This 

 means that the sporogenous tissue 

 is derived from the inner segment 

 following the first periclinal division 

 of the initial. All vascular plants, 

 except the higher ferns, are euspo- 

 rangiate. 



In Phylloglossum the apex of the 

 tuber gives rise to an erect naked 

 stalk bearing a small terminal strob- 

 ilus (Fig. 175). If no strobilus is 

 formed, the stem tip produces a 

 new tuber at the close of the grow- 

 ing season, but otherwise a second- 

 ary tuber appears adventitiously 

 at the end of a short stalk. The 

 strobilus consists of a few spirally 

 arranged sporophylls, each bearing 

 a solitary, unilocular, adaxial spo- 

 rangium. The sporangium is short- 

 stalked and kidney-shaped. It 

 consists of a central mass of sporog- 

 enous tissue surrounded by a wall three layers of cells in thickness, the 

 inner layer forming the tapetum. As in Lycopodium, dehiscence occurs 

 by means of a transverse slit. 



Gametophyte. The spores of Lycopodium are remarkably long-lived 

 and often do not germinate for a number of years. Eventually they give 

 rise to gametophytes, or prothallia as they are usually called. These are 

 small tuberous bodies that vary widely in form, depending on the species, 



Fig. 182. Gametophyte of Lycopodium 

 complanatum. A, the entire gametophyte, 

 twice natural size; B, longitudinal section 

 with anthericUa to the left, some of which 

 have shed their sperms, and with archegonia 

 to the right, in one of which an embryo has 

 developed, X 25. The shaded cells in the 

 lower portion contain a fungus. {After 

 Bruchmann.) 



