410 PLANT PHYSIOLOGY 



From this point of view, plants may be divided into two large 

 groups: those fruiting only once, and those fruiting repeatedly. 

 To the first group belong primarily the annuals, which complete 

 their life cycle within one vegetative period. For the sake of 

 convenience, the cycle of development will be considered not 

 from the moment of embryo formation, but from the time of 

 germination of the seed, the peculiar period of seed development 

 thus being left out of consideration. The following are charac- 

 teristic steps in the life of an annual. The seed germinates; the 

 young plant rapidly develops its vegetative organs, and then it 

 blooms; fruits are set and ripen; finally, all the organs of the plant 

 begin to age rapidly and die; and the plant then dries up. As 

 the period of drying frequently coincides with the autumnal 

 season, it may seem that the death of such a plant is determined 

 by external conditions. In reahty, however, it is dependent 

 upon internal causes. Many early ripening annuals, e.g., the 

 various early varieties of crop plants, produce seeds and dry up 

 long before the end of the growing season. This behavior is 

 still more marked in the ephemerals (Art. 85). 



Blossoming and fruiting do not always occur during the first 

 year. Often it is accomphshed in the second year of the life of 

 a plant. This is true of all biennial plants, such as carrots and 

 beets. They Uve through the winter in a dormant condition, 

 resuming growth only the following spring (Fig. 128). But 

 fruiting of these plants is also followed by death. Finally there 

 are some plants that develop for many years nothing but vege- 

 tative organs, attain a considerable size, and then begin to 

 produce fruit heavily. In such cases, likewise, fruiting is fol- 

 lowed by drying up of the plant. The agave, sometimes called 

 the ''century plant," belongs to this group. In Mexico, its 

 native land, it will begin to blossom at the age of 8 or 10 years. 

 Under less favorable conditions of development, as on the coast 

 of the Mediterranean Sea or in greenhouses, a more protracted 

 period, 20 to 50 years, is required for its development previous 

 to blooming (Fig. 129). Some of the palm trees, among them 

 the famous shade palm of Ceylon {Corypha umbra culifera), 

 display the same characteristic behavior. 



It has not been possible as yet to determine what inner proc- 

 esses cause the dying of plants subsequent to fruiting. But the 

 fact that there exists a close causative connection between the 



