THE ORIGIN OF THE SPOROPHYTE 349 



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

 sporophyte had its origin through the stimulus of the union 

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

 a fusion nucleus with double the number of chromosomes char- 

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

 reduction of this number in many thallophytes before or during 

 the germination of the zygospore or ob'spore, so that there is 

 no opportunity for a sporcphyte generation. This condition has 

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

 true of (Edogonium, Spirogyra, the desmids, Vaucheria, Ulotlirix, 

 and other types. 



The sporophyte arose when nuclear divisions appeared with 

 the double number of chromosomes, thus postponing the time 

 of chromosome reduction to a later period in the life history, 

 which became generally characterized by the formation of 

 asexual spores in tetrads. Sporophytes undoubtedly appeared 

 thus in several groups of plants entirely independently of one 

 another, as illustrated in the divergent lines of development of 

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

 the brown algae), and the bryophytes leading up to the 

 pteridophytes and spermatophytes. 



337. Summary. The alternation of generations in plants 

 takes on added interest when considered in relation to the 

 behavior of the chromosomes, for the importance of the two 

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

 formation- - becomes atTonce apparent. Fertilization doubles the 

 number of chromosomes in the egg and gives it the possibilities 

 of the sporophyte's development. Spore formation reduces the 

 double 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 



V It/ 



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

 the chromosomes hold the rudiments that determine in a broad 



