REPRODUCTION IN PLANTS 



257 



we have not only the beginnings of sex in plants but, in the diploid zygote 

 stage, a foreshadowing of the alternation of generations. 



Reproduction in thallophytes in general. The algae and fungi show 

 a wide range in structure, size, and appearance and have been variously 

 modified for many modes of life. The details of their reproductive processes 

 are correspondingly varied but may be grouped into three main types: 



1. Simple vegetative reproduction, both in unicellular forms, such as 

 the bacteria and certain algae, and 



in many multicellular algae and 

 fungi, where fragments of alga 

 filaments or fungus mycelia grow 

 into new filaments and mycelia. 



2. Spore formation, which may 

 be subdivided into two types: the 

 typical sporulation that follows the 

 production of the diploid zygote, 

 and a type of spore formation that 

 results from the rapid and repeated 

 division of a spore mother cell that 

 was developed from haploid vegeta- 

 tive tissues without fertilization. 



3. Sexual reproduction, which 

 always involves the fusion of two 

 gametes to form a zygote. 



THE ALTERNATION OF 

 GENERATIONS IN THE BRYOPHYTES 



Fig. 17.3. The leafy gametophyte and cap- 

 sule-bearing sporophyte generations of the 

 moss Polytrichum. (Photo by Prof. E. B. 

 Mains.) 



In the mosses and liverworts, 

 which make up the second great 

 division of the plant kingdom, an 

 alternation of gametophyte and 

 sporophyte generations is clear-cut 

 and unmistakable. The gametophyte generation forms the more con- 

 spicuous part of the life cycle and is represented by the small green 

 "leafy" plants that we recognize as mosses 1 or liverworts; but a 

 sporophyte generation is now invariably a part of the cycle. 



Sexual reproduction in the mosses. When the leafy stem of the 

 gametophyte individual has reached its full growth, the upper end 

 produces many-celled sex organs of two kinds. The female sex organs 

 (archegonia) become vaselike in shape, and one of the cells near the 

 bottom of the vase develops into a large egg. The male sex organs (an- 



1 "Spanish moss," so common a sight in the South, is not a moss at all but a flower- 

 ing plant (spermatophyte), related to the air plants and the pineapple. 



