REPRODUCTION 343 



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 so-called 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, thus leaving the peculiar period of seed 

 development out of consideration. The following are character- 

 istic steps in the life of an annual: the seed germinates, the young 

 plant rapidly develops its vegetative organs, then it proceeds to 

 blossom, fruits are set and ripen, and finally all the organs of the 

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

 the period of drying frequently coincides with the autumnal 

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

 determined by external conditions. In reality, however, it is 

 dependent on internal causes. Many early ripening annuals, for 

 instance 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. 



Blossoming and fruiting do not always occur during the first 

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

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

 beets. They live through the winter in a state of rest, resuming 

 growth only the following spring (Fig. 142). 



Fruiting of these plants is also followed by death. There are 

 some plants that develop for many years nothing but vegetative 

 growth, attain a considerable size, and then begin to produce fruit 

 heavily. In such cases, likewise, fruiting is followed 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 blossoming (Fig. 143). 

 Some of the palm trees, among them the famous shade palm of 

 Ceylon (Corypha umbraculifera) 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 two 



