INTRORSE AND EXTRORSE DEHISCENCE 



65 



Development of the Spores. — Inside of the theca, develop certain 

 large cells, in rows, the Spore Mother Cells, each of which, by twice 

 dividing, produces a Tetrad of four pollen-grains. Ordinarily the wall 

 of the mother-cell mostly disappears and leaves the grains separate and 

 mobile, while in other cases they cohere in the tetrad or in a cluster 

 of tetrads. 



Pollinia and Pollinaria. — Large clusters are called Pollinia or Pollen- 

 masses. The entire contents of a theca may form one pollinium (Fig. 

 135), or they may be divided into several (Fig. 140). A cluster of 

 pollinia, like the last, is called a Pollinarium. The number of pollinia 



J3^. 14-0. lU MZ/H. 145. 



Fig. 134. Ring-formed anther of Cyclanthera. 135. Pollinium of Asclepias. 136. Dorsal dehiscence 

 in anther of Hyoscyamus. 137. Dehiscence by apical pores in Menziesia. 138, Dehiscence by valves 

 in anther of Sassafras. 139. Sinuous anther of Sicyos. 140. Pollinarium of 4 pollinia in Ponihiera. 

 141. Marginal dehiscence in anther of Convallaria. 142. Ventral dehiscence in anther of tomato. 

 143. Dehiscence by apical pores in anther of Cassia. 144. Peculiar ventral pores. 145. Apical pores 

 becoming basal by inversion of the anther in Arctoslaphylos. 



in a theca is of much diagnostic importance in the Orchidaceae. The 

 characteristics of the individual pollen-grains are of the utmost value 

 in pharmacognosy, as well as in classification (as, for instance, in the 

 Acanthaceae) , and are discussed in works on histology. 



Dehiscence of the Anther. — We must next consider the structural 

 provisions for permitting the escape of the pollen from the thecae or 

 locellae. This is commonly by splitting, called Dehiscence, along a 

 longitudinal line upon each theca, called the Suture. If the suture is 

 at the back of the anther, as in Hyoscyamus (Fig. 136), the dehiscence 

 is called Dorsal. If upon the face, as in the tomato (Fig. 142), Ventral; 

 if upon the edge, as in Convallaria (Fig. 141), Marginal. 



Introrse and Extrorse Dehiscence. — This suture may face the gynae- 

 cium, when the anther is Introrse by Dehiscence, or away from it, 

 5 



