500 



BOTANY 



PART II 



sporangium consists of a wall enclosing the sporogenous tissue, the 

 cells of which, becoming rounded off' and separated from each other 

 as spore-mother-cells, give rise each by a reduction division to four 

 tetrahedral spores (spore-tetrads). The cells of the innermost layer of 



the sporangial wall are rich in proto- 

 plasm, and constitute the TAPETUM. 

 This layer persists in the Lycopo- 

 dineae, but in the case of the Ferns 

 and Equisetineae the walls of the 

 tapetal layer become dissolved. In 

 the course of the development of 

 the sporangium the tapetal cells 

 then wander in between the spore- 

 mother-cells, their nuclei dividing 

 amitotically, so that the spores 

 eventually lie embedded in a muci- 

 laginous protoplasmic mass, the 



FIG. 465. Venation of Ferns. A, Adiantum 

 capillus veneris (venatio cyclopteridis). B, 

 Asplenium adiantum nigrum (v. spheno- 

 pteridis). C, Asplenium esculentum (v. 

 goniopteridis). D, Polypodium serpens (v. 

 marginariae). E, Polypodium nereifolium 

 (v. goniophlebii). F, Onoclea sensibilis (v. 

 sageniae). 



Fio. 466. Development of the spor- 

 angium of Asplenium. A, First divi- 

 sions of the young sporangium which 

 has originated from a single superficial 

 cell. B, Division into the wall (w), 

 and the central archesporial cell (ar) 

 which has cut off one of the tapetal 

 cells (0- C, Older stage in which the 

 archesporial cell has given rise to the 

 tapetal cells and the sporogenous 

 tissue (sp). (x300. After SAD EBECK.) 



PERIPLASM, from which they derive nourishment ( 114 ). The wall of 

 the mature sporangium is formed of one or a number of layers of 

 cells. The unicellular spores have cell walls composed of several 

 layers. The young spore on becoming isolated in the tetrad surrounds 

 itself with a cutinised membrane (exospore) within which a cellulose 

 layer (endospore) is deposited. In many cases a perispore is deposited 



