342 REPRODUCTION IN PLANTS 



plants which have less effective means of reproduction. 



The Chromosome History in Plants and Animals. 

 — Since the plant chromosomes behave in almost identi- 

 cally the same manner as those of animals there is no 

 necessity to restate the facts, aside from the opportunity 

 it affords to reiterate the essentials of the process and 

 thereby fix them more firmly in mind. It should, however, 

 be remembered, in connection with this statement, that 

 the reduction divisions of plants occur at a different 

 period in the life history (Fig. 95). In animals reduc- 

 tion occurs during the preliminary stages of the formation 

 of gametes, i.e., eggs and sperms, but in plants it occurs 

 just previous to the formation of spores. The spores 

 then develop into gametophytes, large or small according 

 to the sort of plant under consideration. When mature 

 the gametophytes form the eggs and sperms but, of course, 

 no reduction is necessary at this point because it has 

 already taken place in the formation of the spores. The 

 matter may be put another way thus: In the case of the 

 animal the gametes have the reduced number of chromo- 

 somes. Consequently the fertilized egg will have the 

 double number. Since the animal body develops from the 

 fertilized egg by ordinary cell division every cell of the 

 animal body will also have the double number. In the 

 plant the gametes also have the reduced number and the 

 fertilized egg the double number. The fertilized egg 

 grows into the sporophyte plant by cell division and 

 every cell has the double number. Reduction occurs 

 with the formation of spores and therefore the gameto- 

 phyte has the reduced number. From this it will be seen 

 that there is an extra generation — the gametophyte — 

 in the plant life history which has nothing exactly corre- 

 sponding to it in animals. 



Reduction and Segregation of Chromosomes in 

 Plants (Fig. 96). — This process is most frequently 

 studied in the anther because there are many spore 

 mother-cells giving an opportunity to see many different 



