442 PLANT PHYSIOLOGY 



cultivated under irrigation, after an abundant irrigation in the 

 beginning of summer, which promotes profuse flowering; if the 

 plants remain for a long time without irrigation; or if a very 

 severe drought occurs. In general, any abrupt change in the 

 conditions of growth, especially if this change is disadvantageous 

 and occurs during setting of the fruit, may lead to their abscission. 

 Careful attention must be given to the plant at this sensitive 

 period of life. Some fruits, such as peaches, plums, apples, and 

 pears, have a periodicity in the shedding of their immature fruits 

 that seems to be of a hereditary nature (Detjen, Murneek). 



101. Ripening of Fruit and Seeds and the Biochemical Proc- 

 esses Accompanying It. Artificial Acceleration of Ripening. — 

 Ripening of fruit and seeds is not merely the attainment of the 

 proper size characteristic of a given variety of plants. A series 

 of complicated morphological and biochemical changes occur in 

 the fruit during ripening. The final stage of development of 

 seeds is the attainment of dormancy, in which they may remain 

 for a prolonged period until they are placed in conditions favorable 

 for germination. In most plants, ripening of the seeds is accom- 

 panied by their gradual desiccation until they reach an air-dry 

 state. The other parts of the fruit play only an auxiliary role 

 in the process of reproduction. In dry fruits, they serve for the 

 protection of the seeds. In succulent fruit, they attract animals, 

 which consume them and thus contribute to the dispersal of the 

 seeds, which in many cases are provided with a hard seed coat 

 that allows them to pass through the alimentary tract of animals 

 uninjured. 



From a morphological viewpoint, ripening of the seeds repre- 

 sent the embryo, beginning from the fertilized egg cell and con- 

 tinuing till it becomes a small plantlet with its basic embryonic 

 organs, the rootlet, the cotyledons, and the growing point of the 

 stem. At the same time, food reserves are accumulated, which 

 are necessary for the first stages of development of the young 

 seedling before it has begun independently to absorb and assim- 

 ilate nutritive substance from the surrounding medium. Their 

 accumulation in the seeds will not be examined. The char- 

 acter of these reserves and their metaboUsm have already been 

 discussed. 



Fundamentally, the accretion of reserves in the ripening seeds 

 represents somewhat the reverse of the processes occurring during 



