ELEMENTARY STRUCTURE. 



61 



commonly takes place in the formation of pollen, is as follows :-— 

 {Jig. 144) ; the primordial utricle of each parent cell becomes 

 infolded as in ordinary cell- division, so as to divide it into four 

 portions, either directly, or indirectly by first dividing it into two, 

 and then each of these being again divided into two others ; these, 

 four portions are called special parent cells ; the whole of the 

 protoplasmic contents in each of these then secrete a layer of 

 membrane on their outside, and we have thus four perfect cells 

 formed in the cavity of their parent. As these continue to 

 enlarge, the walls of the parent cells, and subsequently those of 

 the special parent cells, become in most cases absorbed, and the 

 cells being thus set free, the process is completed. The manner 

 in which the spores are formed in the higher Flowerless Plants is 

 substantially the same in most cases. It sometimes happens, how- 

 ever, that in the development of pollen and spores, the special 

 parent cells are not formed, as has been sho-\\Ti by Schacht in the 

 pollen of (Enothera, and in the spores oi Anthoceros Icevis ; and by 

 Henfrey in the spores oi Marchantia polymorpha {fig. 145). 



Fig. 14; 



Fig. 146. 



Fig. 145. a. Cylindrical cell from which are formed the parent cells of the 

 spores oi Marchantia polymorpha; p. Primordial utricle of the parent 

 cells. 6. The same cell converted into a string of cells, c. One of the 

 parent cells isolated, -with four primordial utricles of the spores, d. The 



four spores free- After Henfrey Fig. 146. Formation of zoospores iu 



Achhja prolifera. After Carpenter. 



In some cases, instead of the development of only four second- 

 ary cells in the cavity of the parent, avc have a large number 

 formed in this manner {fig. 146, a), which either escape {fig. 

 146, b) from it clothed by a cellulose coat, as is ordinarily the 

 case, or this is secreted after their separation from the parent 



