THE FLOWER AND FRUIT 97 



Each petal and stamen is ordinarily supplied by a single bundle, 

 and the number of major bundles supplying a carpel is normally 

 three, one abaxial and two adaxial. These are frequently modified, 

 especially in compound pistils where adjacent adaxial bundles may 

 anastomose. In some instances, the abaxial bundle may appear to 

 be a double bundle (Pisum), owing to the development of a bisect- 

 ing parenchymatous ray. 



MicROSPOROGENESis. — The initial protuberance of the primor- 

 dium of a stamen develops into two parts, the terminal anther and 

 the filament. In the anther, the cells are at first uniform, except 

 for the epidermis; but, a little later, the cells of the provascular 

 strand are differentiated, and a hypodermal cell or a group of them 

 appear which have a denser cytoplasm and can be distinguished 

 from the adjacent cells on this basis. These are the archesporial 

 cells, although the whole hypodermal layer may be regarded as 

 having archesporial potentialities. The number of cells which are 

 functional is ordinarily limited, and their subsequent development 

 results in the formation of the lobed regions of the anther. They 

 occur in longitudinal rows; and, in a transection of the anther, may 

 appear as a single cell or as groups of four to six or more. Cases 

 have been reported in which a single cell may constitute the whole 

 archesporium. 



The archesporial cells enlarge and divide both radially and 

 tangentially. The tangential divisions result in the differentia- 

 tion of outer cells as the primary -parietal cells and the inner as 

 the primary sporogenous cells. From the former and their deriva- 

 tives, the walls of the embedded sporangia, of which there are typi- 

 cally four, develop. If the cells of the primary parietal layer 

 divide periclinally, a series of parietal wall layers of varying number 

 is developed. The outermost wall layer constitutes the endothecium. 

 It may become definitely specialized by the development of wall 

 thickenings which are related to the dehiscence of the anther. 



The innermost parietal layer or sometimes the outer sporogenous 

 layers form the tapetum. Regardless of its morphological origin, 

 the tapetum serves as a nutritive layer for the developing microspore 

 mother cells and microspores., and it is more or less completely digested 

 during the progress of their growth and maturation. During the 

 development of the primary parietal cells, the primary sporogenous 

 cells may undergo successive divisions or function directly as micro- 

 spore mother cells. In either case, each microspore mother cell 



