894 ECOLOGY 



The above experimental evidence compels the abandonment of the 

 notion that only the usual succession of events in a plant in nature is 

 to be considered normal. One thing alone is fixed, namely, that plants 

 must at the outset have a period of vegetative activity, but whether this 

 continues through life, or whether the reproductive period begins early 

 or late or not at all, is a matter that is determined by external conditions, 

 and one series of events is quite as normal as another. It is quite as 

 normal for a bulrush to live in deep water and to vegetate indefinitely 

 as to live in shallow water and to flower annually. Indeed it is permis- 

 sible to regard anything that a plant ever does as normal, since in every 

 case its particular reactions are due to its life conditions. Thus it is 

 demonstrated that plants as a rule do not possess an inherent rhythm, 

 since external factors are the dominant determining causes of reproduc- 

 tive periods; if plants flower regularly, the requisite conditions may 

 be supposed to recur regularly. 



The exact analysis of the reproductive factors remains to be deter- 

 mined, though the data now available are sufficient to show that suc- 

 cessive stages imply successive causes (i.e. changed conditions), while 

 uniform phenomena imply uniform or unchanged conditions. Broadly 

 speaking, the conditions commonly termed hydrophytic and mesophytic 

 seem especially to favor the continuance of vegetative activity, while 

 xerophytic conditions favor reproduction. Hence it is not unlikely that 

 the amount of available water may be a dominant specific factor, es- 

 pecially as its presence in abundance is known to be a primary requisite 

 for optimum vegetative development. 



Another important reproductive factor is light. Careful experiments on Mimulus 

 show that light of high intensity favors flower production. In the giant cactus 

 there is a ring of flowers about the stem, and anthesis begins on the side toward the 

 sun. In the teasel ( Dipsacus) there is a tendency for the basal flowers to blossom 

 first, but usually the lighting is better toward the upper part of the inflorescence; 

 as a resultant of the two factors concerned, the first flowers to appear usually are 

 those near the middle. High temperature also favors flowering. It has been shown 

 that if radishes are grown in concentrated (10 per cent) solutions of glucose, they 

 bloom earlier than otherwise; this may explain why light favors flowering, since 

 it facilitates the production of carbohydrates; also the girdling of trees, which hastens 

 flowering, would tend to cause the accumulation of carbohydrates in the upper parts 

 of the plant. It has been claimed that a minimal supply of food salts (especially 

 nitrates and phosphates) at times favors flower production. Defoliation occasioned 

 by storms, by insects, or by freezing sometimes causes flower production, but the 

 reason is not obvious. The influence of parasites upon flower production varies; 

 the black rot appears to stimulate autumnal flowering in the apple ; in other cases 



