38 



MORPHOLOGY OF ANGIOSPERMS 



D 



coincident with the mother-cell stage, but the greatest devel- 

 opment of the tapetal cells is during the formation of tetrads. 

 During this process they may increase greatly in size, this being 

 associated with the disorganization of the cells of one or more 

 of the middle layers. It is very common for the enlarged 

 tapetal cells, filled with food material, to become binucleate 

 (Fig. 10, C), and later even multinucleate, as in Typha (Schaff- 



ner 17 ) and llepatica (Coul- 

 ter 19 ), in the latter genus 

 six to thirteen nuclei hav- 

 ing been observed in a sin- 

 gle cell. At the end of the 

 tetrad division the tapetal 

 cells usually become disor- 

 gaiiize-d v - also such of the 

 middle layers as have not 

 disorganized previously, and 

 - the outermost parietal layer 

 begins to develop the thick- 

 enings characteristic of the 

 endothecium. The fact that 

 the endothecium may con- 

 sist of additional layers of 

 cells has already been men- 

 tioned. 



During the development 

 of the parietal layers the 

 primary sporogenous cells 

 either directlv or bv divi- 

 sion produce the mother- 

 cells. When division oc- 

 curs, it is in every direc- 

 tion, so that all appearance 

 of layers is lost. Perhaps 

 the usual case is for the primary sporogenous cells to divide two 

 or three times, but there are sometimes more divisions, and a 

 number of cases are known in which the primary sporogenous 

 cells, without division, become mother-cells, as has been long 

 known in Malva, Datura, Mentha, and Chrysanthemum, and 

 recently reported for several species of Asclepiadaceae by Stras- 



Fig. 13. — A and D, Or-chis maculata : A, trans- 

 verse section of an anther with four micro- 

 sporangia, each showing five or six cells, 

 each of which gives rise to a " massula" as 

 shown in D. B, C, and £, Neottia ovata : 

 B, a tetrad ; C, the four microspores within 

 the common wall dividing to form tube nu- 

 cleus and generative cell ; £, the division 

 completed ; two of the microspores show the 

 generative cell cut off by a lenticular wall. 

 A x 2o\ D x 240; B, C, E x 305.— After 



GuiONARI). 10 



