136 ALTERNATION OF GENERATIONS 



form the food in the endosperm and it also functions as an 

 absorbing organ of the embryo (Fig. 86, D). Similar devices 

 for absorbing the storage foods and transferring them to the 

 embryo are seen in the wheat, onion, date, and cocoanut. The 

 early growth of the embryo goes on as in the preceding examples. 

 The root first emerges and anchors the grain in the soil. The 

 plumule of closely rolled leaves forms a sharp bodkin that reaches 

 up through the soil with ease. This modification of the plumule 

 would appear to be of a decided advantage and you will notice 

 that this method of emerging from the soil is followed by a great 

 many plants with underground stems ; see the large leaves of the 

 skunk cabbage, may apple, Solomon's seal, etc. The rest of 

 the grain remains in the ground as in the case of the pea. 



57. The Two Phases in the Life of a Plant. — As has been 

 stated the plumule unfolds in all these cases like a bud and 

 gradually develops into the mature plant. This may be effected 

 in one -year or several years may be required to complete the 

 work. At some time in this growth flowers will appear bearing 

 sporophylls and spores and thus we arrive again at the starting 

 point in the life history of a plant. There are two distinct phases 

 or generations in the life of a plant. The formation of the 

 megaspores and microspores marks the beginning of the sexual 

 generation or gametophyte which ends when the male and female 

 gametes have been developed. The formation of the gameto- 

 spore marks the beginning of the asexual generation or sporo- 

 phyte which ends in the sporophylls when those cells, called the 

 spore mother cells, appear which produce the micro- and mega- 

 spores. These two generations are also sharply distinguished 

 from each other by the number of chromosomes, page 56, which 

 their cells contain. The cells of the gametophyte generation 

 contain only one-half the number of chromosomes found in the 

 cells of the sporophyte generation. If the nuclei of male and 

 female gametes contain twelve chromosomes (the number varies 

 in difi'erent plants) the gametospore which marks the beginning 

 of the asexual generation, will contain twenty-four chromosomes. 

 This number will be found in all the cells of the embryo and 

 continue to characterize the subsequent development of the sporo- 



