THE MALE GAMETOPHYTE 



175 



Scirpus (Fig. 60). The formation of this first cell as a wall-less, free 

 protoplast probabl)' marks a forward step in the reduction of the 

 gametophyte. Though definite positions in the spore have been claimed 

 for the first walled cell — and there may be a constant position in a 

 given species — several or many diflFerent positions have been reported 

 for angiosperms as a whole. The first-formed cell, walled and peripheral, 



Fig. 70. Diagram showing development of male gametophyte in angiosperms. A, 

 microspore; B, microspore, enlarged, vacuolate; C, spore nucleus dividing; D, ga- 

 m.etophyte, two-celled, smaller generative cell, larger vegetative (tube) cell; E, 

 freeing of generative cell from spore wall; F, generative cell embedded in vegetative 

 cell; G, H, di\'ision of generative cell to form two male gametes within the spore 

 wall; I, J, division of generative cell within the pollen tube. (After Maheshwari.) 



is the generative cell (Figs. TOD and 71G); the larger, naked cell, cen- 

 tral and wall-less, filling the remainder of the spore-wall cavity, is the 

 vegetative or tube cell. The nuclei of these two cells differ not only in 

 size but in structure and staining qualities. The vegetative nucleus has 

 a prominent nucleolus; the generative nucleus, a small nucleolus or 

 none. The generative cell is soon freed from the spore wall and be- 

 comes ellipsoid or fusiform. Prominent cytoplasmic differences set it 



