REPRODUCTIVE ORGANS. 



Ill 



of the moss-plants is still more true 

 of the fern-plants ; while among 

 the seed-plants certain spores (pol- 

 len grains) are conspicuous (see 

 Pig. 110), but the eggs can be ob- 

 served only by special manipulation 

 in the laboratory. Seeds are neither 

 spores nor eggs, but peculiar repro- 

 ductive bodies which the hidden 

 egg has helped to produce. 



73. Germination. — .Spores and 

 eggs are expected to germinate ; 

 that is, to begin the development 

 of a new plant. This germination 

 needs certain external conditions, 

 prominent among which are defi- 

 nite amounts of heat, moisture, 

 and oxygen, and sometimes light. 

 Conditions of germination may be 

 observed most easily in connection 

 with seeds. It must be understood, 

 however, that what is called the 

 germination of seeds is something 



very different from the germination 

 of spores and eggs. In the latter 

 cases, germination includes the very 

 beginnings of the young plant. In 

 the case of a seed, germination begun 

 by an egg has been checked, and 

 seed germination is its renewal. In 

 other words, an egg has germinated 

 and produced a young plant called 

 the "embryo," and the germination 

 of the seed simply consists in the 

 continued growth and the escape of 

 this embryo. 



Fig. 108. A portion of the 

 body of a common alga 

 {CEdogoniiim), showing 

 gametes of very unequal size 

 and activity ; a very large 

 one io) is lying in a globular 

 cell, and a very small one is 

 entering the cell, another 

 similar one (s) being just 

 outside. The two small 

 gametes have hair-like pro- 

 cesses and can swim freely. 

 The small and large gam- 

 etes unite and form an egg. 



Fig. 109. A group of swim- 

 ming cells : ^, a spore of 

 Oidorjoiihim (an alga) ; 

 5, spores of UlnHirix {axi 

 alga) ; C, a gamete of 

 Equiseium (horse-tail or 

 scouring rush). 



