50 Reproduction and Meiosis 



the parents have the diploid number. A succession of events, 

 starting with one stage of life and including all the steps that 

 occur until a new individual at the same stage as the first is 

 found, is called the life cycle of the organism. The life cycle 

 of most animals, including the vertebrates, is very simple. 



Reproduction and Life Cycles in Higher Plants 



In plants above the Thallophytes and, indeed, in many algae 

 and fungi, reproduction is complicated by a more involved life 

 cycle than is generally found in the Animal Kingdom. Although 

 we frequently think that we can recognize the body of a certain 

 kind of plant, few of us except botanists recognize that in all 

 these higher plants the complete life cycle includes two different 

 plant bodies. In the fern plant, for example, we are all familiar 

 with the often large, leafy structure that bears typical "fern 

 leaves." These leaves bear minute spores in clusters on their 

 under side. When these spores germinate, they do not produce 

 typical fern plants but small, flat, green bodies, perhaps half an 

 inch long or less, which lie close to the ground. These bodies 

 are fern plants just as much as the more familiar types and 

 they bear the gametes. When the gametes unite, a zygote is 

 produced which develops into the familiar type of fern plant. 

 Thus the complete life cycle of a fern includes two bodies: the 

 large body on which these spores are found, the sporophyte, and 

 the small body that bears the gametes, the gametophyte. 



The meiotic divisions in the fern occur not in the formation 

 of the gametes, as in animals, but in the development of the 

 spores. The sporophyte plant has 2n chromosomes and pro- 

 duces haploid spores. They germinate and by a series of regular 

 mitotic divisions produce the haploid gametophyte, which in 

 turn produces haploid gametes. They unite to form a diploid 

 zygote, which, in turn, develops into the diploid sporophyte 

 body. This alternate production of sporophyte and gameto- 

 phyte bodies is called alternation of generations. 



The existence of two generations in the life cycle of the higher 

 plants can best be grasped from such a plant as the fern, where 

 each generation is throughout most of its life a separate and 

 independent structure. In the seed plant the gametophyte is 

 reduced in size and complexity to only a few cells. 



