13 



form with rounded corners indiaiting the positions of the 

 four microsporangia. It shows in longitudinal section some 

 I'0-12 rows of periblem cells, and in transxerse section 6-8, 

 with a central plerome initial of the connective. Beneath 

 the epidermis at each of the four corners the archesporial 

 cells are now mapped out, and differentiation of the 

 sporangia proper begins from these specialised hypodermal 

 cells. A cross-section gives one archesporial cell only for 

 each sporangium, as Cannon^ found in A:'L'na fatua, the 

 wild oat ; in vertical section three or more are shown. 

 ICach archesporial cell enlarges and divides (Fig. 6) uito 

 an outer primary parietal or wall cell and an inner 

 primary sporogenous cell. The parietal cells undergo 

 two successive divisions (Fig. 7), resulting in the formation 

 of the sporangium wall, consisting of three distinct layers 

 of cells, endothecium, middle layer, and the innermost 

 tapetum a jacket of nourishing cells filled with dense 

 granular cytoplasm in which 1-2 nuclei are embedded. 



Meanwhile the primary sporogenous cells become 

 deep-seated and active ; their nuclei, passing through two 

 or three mitoses, form a central rounded mass of poly- 

 hedral sporogenous cells, radially arranged and surrounded 

 by the wall la}-ers, the whole being (-oxered b)- the 

 exothecium or epidermi.s. 



Movements within the nuclei of the sporogenous cells 

 indicate their transformation directly into the so-called 

 microspore mother-cells. By the rounding off and separa- 

 tion of their walls, the mother-cells come to lie free in the 

 loculus of the Microsporangium (Fig. <S) as large rounded 

 or oval cells, each having a central vacuole containing the 

 now free nucleus. This mother-cell condition may be 

 retained for some time, and has been termed the " resting " 

 stage. It is reached about the time the archesporium 

 makes its appearance in the mcgasporangium ; synapsis 



