514 MANUAL OF BOTANY 



limits. The various names given to them, such as ascogonidia^ 

 stijlogonidia, teleutogonidia, indicate various conditions of this 

 kind, and not any difference in tlieir own structure. As a rule, 

 they are small spherical or flattened bodies, clothed with a cell- 

 wall ; in both Algge and Fungi cases occur where they have no such 

 coating and are able to swim about by means of cilia. These 

 forms are called zoospores, or zoogonidia. 



In the higher plants, in which they originate only on the 

 sporophyte, though possessing fundamentally the same structure 

 as in the lower ones, their walls show more differentiation, 

 possessing two coats, and often having the outer one curiously 

 sculptured or furnished with projecting ridges or spines. Some 

 have a third coat derived from the disintegrated mother cells 

 and tapetum. 



In some Pteridophyta an important variation makes its 

 appearance ; the spores are no longer all alike, but two kinds 

 are produced, the microspores and the macrospores. This 

 phenomenon is known as heterospory, and it extends also 

 throughout the next group, the Phanerogams. 



The microspores have the structure already described ; 

 the macrospores in the Pteridophyta are much like the micro- 

 spores, except in size ; in the Phanerogams they are simpler, 

 having, with the exception of those of the Cycads, a single wall 

 which remains thin and delicate. In this group too the macro- 

 spore is never set free from the sporangium, nor is the latter 

 detached from the plant mitil after the germination of its spore. 

 Tiie body produced from the germinating spore or gonidium is 

 always a gametophyte, though often it only produces other gonidia. 

 In the second case in which there is a production of special 

 reproductive cells, these are incapable alone of producing a new 

 individual, but must fuse together m pairs to bring about this 

 result. They are consequently called serual cells or gametes. 

 Tliese probably took their origin from the asexual form. The 

 p lase of the plant on which they occur is always the gameto- 

 phyte. The latter can consequently give rise to either sexual or 

 asexual cells or both, while the sporophyte can only produce the 

 latter. 



In all the higher plants the gametophyte and sporophyte 

 regularly alternate with each other, each producing the other 

 phase of the life cycle. This phenomenon is known as anti- 

 thetical alternation of generations. In the lower forms it is not 

 at all regular in its occurrence ; the sporophyte being altogether 

 unrepresented in some, and only occurring at intervals in others. 



