10 MANUAL OP BOTANY 



it bears only spores, is known as the sporophyte, or spore-bearing 

 generation. The other form, a little green flattened body, called 

 the 2}rothaIlus, produces sexual oigaxis, which give rise to repro- 

 ductive cells of two kinds, neither of which can by itself give rise 

 to a new organism, but after a process of fusion of two of them, 

 one of each kind, the resulting cell can originate such a develop- 

 ment. From the spore we always find a prothallus springs, and 

 from the fused gametes of the latter, which unite to form a, cell 

 called the oospore or zygote, the new Fern arises. 



In some plants the sexual cells are apparently all alike, but 

 in most tliey differ in shape, size, power of motion, &c. Whether 

 similar or dissimilar they are known as gametes, and the phase of 

 the plant which bears them is consequently called the gameto- 

 phyte. 



In many plants the sporophyte produces two kinds of spore, 

 each of which gives rise to a special form of gametophyte. In 

 such cases the two kinds of spore differ very greatly in size. 



In all the higher plants the two forma regularly alternate 

 the sexual cells of the gametophyte giving rise to the sporophyte, 

 and the spores of the latter reproducing the gametophyte — the 

 life history of each plant so exhibiting what is known as an alter- 

 nation of generations. 



In the lower forms, among the Algaj and Fungi, another 

 variety of alternation is met with. Their gametophytes, in many 

 cases, bear asexual cells as well as gametes, and frequently the 

 gametes are not developed, so that many asexual generations 

 often foUow each other before a sexually produced form recurs. 



The asexual cells which arise upon the gametophyte are 

 called gonidia. 



Of the two forms described, the gametophyte is usually the 

 more prominent among the lower plants. In the Mosses the 

 two are almost equal in degree of development. The gameto- 

 phyte is the so-called moss plant, and consists of an axis fur- 

 nished with numerous leaves. The sporophyte consists of a 

 swollen head or capsule, supported on a stalk of variable length, 

 and remaining attached to the gametophyte. It forms the body 

 which is often somewhat loosely spoken of as the fruit. 



Above the Mosses the tendency is to greater development of 

 the sporophyte and retrogression of the gametophyte. In the 

 Ferns and the higher Cryptogams the plant known under the 

 ordinary name of Fern, Horsetail, or Club-moss is the sporo- 

 phyte, and the gametophyte is a small structure, consisting of only 

 a plato of cells, or a small tuber-Hke mass, and showing little or no 



