REPRODUCTIVE CEFiS. 119 



only) and Heterosporous (two kinds of spore), and both will be 

 studied on account of the valuable comparisons which can be 

 made between their reproductive cycles and those which occur 

 in the Angiosperms and Grymnosperms. 



After reproduction in the higher types of plant has been 

 studied, a brief description will be given of the process as it 

 occurs in the Bryophyta, Fungi, and Algae. It is not intended 

 to examine very fully reproduction in the Fungi, so that only 

 an outline of this will be given. The important point is to gain 

 a clear idea of the reproductive cycles met ^vith in the three 

 great groups mentioned above. 



A. Reproduction in Angiosperms. 



In the Angiosperms the essential primary sexual elements are 

 the following : — 



a. The microspore or pollen-grain. 



b. The macrospore or embryo-sac. 



Each of these undergoes a process of maturation, and at the 

 completion of this fertilisation takes place. 



a. The , Microspore : its Origin and Maturation. — The pollen- 

 grains are produced in certain parts of the anthers of a flower ; 

 usually four rudimentary masses of cells are set off in the young 

 anther, and these are the archesporial cells. These cells arise by 

 the division of a primary archesporial cell, and of the resulting 

 mass the outermost cells give rise by further divisions to a 

 sheathing layer known as the tapetum ; the remaining inner 

 mass form the pollen mother-cells, from which the microspores 

 are ultimately produced (see Fig. 91, a). 



At a somewhat later period each mother - cell undergoes 

 division into two cells, the resulting cells dividing again (see 

 Fig. 91, b, c, d), so that ultimately there are four nucleated 

 masses of cytoplasm enclosed within the original wall of the 

 parent-cell. Each of the four masses is a potential microspore, 

 and soon assumes a thin wall of cellulose which, later on, 

 becomes modified in a manner which will be described. A large 

 number of mother-cells are often present, and it will be seen that 

 the number of microspores ultimately formed is four times as 

 great as that of the mother-cells. 



