346 TEXTBOOK OF PLANT PHYSIOLOGY 



respects excepting for an insufficiency of illumination, the blooming 

 will be considerably delayed and sometimes does not occur at all. 

 Klebs, for instance, grew in diffused light specimens of Glechoma 

 hederacea, which remained in a vegetative condition for several 

 years. But when the plants were placed in a stronger light they 

 soon began to blossom. Similar experiments have been successful 

 with other plants also. 



This effect of light, however, is more of an indirect than that 

 of direct nature. Plants that have a considerable storage reserve 

 of organic substances, especially carbohydrates, will blossom even 

 in darkness. The hyacinth and other bulbous plants are good 

 examples of such a behavior. Similarly, it is possible to obtain 

 flowers on separate branches of a plant, which are placed for 

 this purpose in a dark chamber, providing their organic connection 

 with other parts of the plant remain intact and the rest of the plant 

 is exposed to strong light. On the basis of these facts Klebs was 

 led to the idea that light hastens the period of blossoming chiefly 

 by assisting the plant to accumulate carbohydrates and that the 

 latter is the direct cause of the change from the stage of vegetative 

 growth to that of reproduction. 



This has been confirmed by experiments with plants grown 

 under different kinds of colored glass, each representing a certain 

 part of the spectrum. In red light, plants flower almost as readily 

 as in white light, while blue light delays blossoming, despite the 

 fact that the formative effect of light depends chiefly on the pres- 

 ence of blue rays. But analyses have shown that plants grown in 

 red light are much richer in carbohydrates, both starch and sugars, 

 than those grown in blue light. 



Besides the carbohydrates, the amount of mineral substances 

 and various nitrogen compounds also are of importance in deter- 

 mining the time of fruiting. It has been noted for a long time that 

 nitrogenous fertilizers increase the vegetative growth of a plant at 

 the expense of its reproductive functions. All the other soil 

 nutrients seem to have a similar effect, with the exception of phos- 

 phoric acid, which usually increases fruiting. According to Klebs 

 it is not the absolute amount of particular substances which deter- 

 mines the time of fruiting, but their relation to each other. A 

 preponderance of carbohydrates over nitrogen and mineral com- 

 pounds leads to flowering and fruiting; a reverse relationship, to 

 vegetative development; hence, by combining certain fertilizers 



