REPRODUCTION 351 



country by an abundant tuber formation, like wild species of 

 potatoes, Oxalis, etc., produce no tubers at all when grown in cen- 

 tral Europe with its longer summer day. But by shortening the 

 day artificially to 12 hr., numerous tubers are obtained. In the 

 cultivated potato, tuber formation is also stimulated by a short 

 day. For this reason it is especially intensive in autumn. In 

 summer, with its long day, it is chiefly the vine which develops. 

 As the production of a sufficient amount of starch requires a large 

 leaf surface, the northern latitudes present especially favorable 

 conditions for potato cultivation. During the northern summer, 

 with its long day, the organs of assimilation develop vigorously, 

 after which, with the considerably shorter day in autumn, a large 

 amount of reserve substances is quickly accumulated in the tubers. 



More recent investigations have shown that in photoperiodism 

 the same after effect may be observed as under the influence of a 

 lowered temperature. Thus, if the young shoots of millet, a short- 

 day plant, are kept several days under the conditions of a long 

 day and are afterwards exposed to a short day, it will be observed 

 that the time of flowering is thereby considerably delayed, in 

 comparison to plants grown continuously under short-day con- 

 ditions, and that the vegetative organs have become strongly 

 developed. The phenomenon of photoperiodic after effect makes 

 us suppose that already in very early stages of development, the 

 length of the day is able either to stimulate or to delay the forma- 

 tion of hormones specific for flowering, and that the subsequent 

 course of development is to a considerable extent determined by the 

 amount of hormones accumulated. 



101. The Internal Factors of Fruiting.— External conditions 

 do not directly produce or delay fruiting. They merely modify 

 the internal condition of the plant, which, in turn, will produce 

 either vegetative growth or reproductive organs. Even such a 

 factor as the relationship between carbohydrates or nitrogen-free 

 organic substances, and the amount of nitrogenous compounds and 

 mineral salts, which is more of an internal than an external factor, 

 cannot be regarded as an immediate cause of fruiting. 



One of the most important inner causes determining the time 

 of fruiting is the age of the plant. Many plants, particularly 

 annuals, will begin to flower within 3 to 4 weeks after sowing, while 

 some species of trees attain maturity only after several years. 

 Thus, for instance, some oaks, elms, and chestnuts will not pro- 



