THE ORIGIN OF THE SPOROPHYTE 349 



336. The origin of the sporophyte. It seems clear that the 

 )rophyte had its origin through the stimulus of the union 



)f gametes, and especially the union of gamete nuclei, to give 

 fusion nucleus with double the number of chromosomes char- 

 steristic of the gametophytes. It is probable that there is a 

 eduction of this number in many thallophytes before or during 

 germination of the zygospore or obspore, so that there is 

 10 opportunity for a sporophyte generation. This condition has 

 jen reported for Coleochcete (Sec. 222), and it is probably also 

 le of CEdogonium, Spirogyra, the desmids, Vaucheria, Ulothrix, 

 id other types. 



The sporophyte arose when nuclear divisions appeared with 

 double number of chromosomes, thus postponing the time 

 chromosome reduction to a later period in the life history, 

 rhich became generally characterized by the formation of 

 3xual spores in tetrads. Sporophytes undoubtedly appeared 

 ms in several groups of plants entirely independently of one 

 lother, as illustrated in the divergent lines of development of 

 red algae, the sac fungi, the Dictyotacece (a small group of 

 le brown algse), and the bryophytes leading up to the 

 ^ridophytes and spermatophytes. 



337. Summary. The alternation of generations in plants 

 ikes on added interest when considered in relation to the 

 jhavior of the chromosomes, for the importance of the two 

 ritical stages in the life history (1) fertilization, and (2) spore 



formation becomes at once apparent. Fertilization doubles the 

 lumber of chromosomes in the egg and gives it the possibilities 

 the sporophyte's development. Spore formation reduces the 

 louble number of chromosomes by half and brings the plant's 

 protoplasm back to the condition where it may develop the 

 gametophyte. The two processes follow one another as the life 

 history is repeated again and again with machine-like regularity, 

 and there is undoubtedly a chemical and physical basis for the 

 life history. And, as before stated, it is generally believed that 

 the chromosomes hold the rudiments that determine in a broad 



