412 PLANT PHYSIOLOGY 



Other types of plants are characterized by repeated blooming. 

 Here belong the great majority of perennials and woody plants. 

 They blossom and fruit every year. Still, that does not prevent 

 their further vegetative growth. The duration of their life is 

 practically unlimited. What inner physiological readjustments 

 enable them to continue their vegetative growth after fruiting 

 are at present unknown. In some plants like the tomato and 

 cucurbits, alternate cycles of growth and fruiting may follow 

 each other in successive order. The contents and requirements 

 of the fruit seem to control growth in such plants (Murneek). 



Although the accomplishment of its life cycle is usually accom- 

 panied by the growth of the plant, nevertheless these two phe- 

 nomena are far from being identical. Growth basically consists 

 in an increase in size and the repeated formation of vegetative 

 organs, stems, leaves, and roots. The chief function of these 

 organs is participation in the further accumulation of organic 

 substances of the plant. The most important symptom of the 

 development of a plant, its transition to sexual reproduction, 

 must be regarded as the origin of a new quality, viz., the ability 

 of the plant to reproduce by seed, occurring as a result of a series 

 of quantitative changes. As has been shown by the investiga- 

 tions of Lysenko, the transition of the plant to the reproductive 

 phase occurs not suddenly but gradually by a sequence of stages 

 of transition through definite stages of development, terminating 

 in the formation of ripe seed similar, but not identical, to the 

 seed from which the plant was originally developed. By sexual 

 propagation alone, the individual dies, but the species continues 

 to exist and to develop in the alternation of generations. 



95. Conditions for the Formation of Reproductive Organs. 

 Hypothesis of Klebs of the Significance of Changes in the Chem- 

 ical Composition of Plants. — Definite conditions must be realized 

 for the transition of a plant from the vegetative to the reproduc- 

 tive phase. Without these conditions, the transition to repro- 

 duction is impossible, and the plant remains sterile during an 

 indefinite period. The understanding of these conditions is of 

 great theoretical and practical importance; for if these conditions 

 are known, the development of the plant may be influenced in the 

 desirable direction, i.e., the cycle of the development may be con- 

 trolled by changing the conditions. As has been seen, such a 

 ( ontrol represents one of the chief problems of plant physiology. 



