GENERAL CONCLUSIONS AND SUMMARY OF STUDIES. 4 1 



the seedlings appeared no more water was given the pot until February 2, 

 when by wilting in the daytime the plants showed that more was required. 

 On February 2 the earth of the pot was well watered. Very shortly after 

 the watering, the plants recovered and began growing vigorously, and on 

 February 9 they were removed from the pot and their roots were examined. 

 It was learned that the main root bore laterals of the first order singly ; no 

 rudimentary roots were present. It would therefore appear that in Rafin- 

 esqiiia not only are rudimentary roots not present in the natural condition, 

 but their formation may not be induced by improved water relations. 



The root-systems of the winter annuals are usually easily distinguishable 

 from those of the summer annuals. The most striking characteristics of the 

 roots of the former are the prominently developed tap root and the meager 

 development of the laterals, which are generally filamentous or at least thin. 

 The annuals of summer, on the other hand, have root-systems which are 

 frequently of a more generalized form ; that is, the laterals are developed 

 well and are frequently rather coarse, and the main root is often forked. 

 The absorbing surface of the summer annuals appears to be greater than that 

 of the winter forms, although the depth of penetration of the roots of the two 

 classes is apparently about the same. 



The causes leading to the differentiation in the root-systems of the win- 

 ter and the summer annuals are not surely known, but on the probability 

 that they lie in the nature of the species as well as in the difference in the 

 environment of the two classes of plants the following hypothesis, as a basis 

 for subsequent experimentation, is offered. The facts in addition to those 

 presented in the foregoing paragraph arc as follows. The rains of summer, 

 in addition to thoroughly wetting the soil, serve to cool it as soon as they 

 come. The air temperature also immediately falls, and the relative humid- 

 ity at once becomes high. In brief, the conditions for a tropical luxuri- 

 ance of growth are at hand, and the shoots of the plants which appear at 

 this season bear a noticeably large number of large leaves ; the transpiration 

 surface is relatively great. 



In the winter season, however, the general character of the annuals, and 

 that of their environment, are strikingly different from those just described 

 as obtaining in summer. As was shown in the discussion on soil temper- 

 atures, the temperature of the soil begins to fall with the coming of the 

 summer rains and continues to decline until March-April. Therefore, the 

 rains of the winter season do not materially change the course of the curve, 

 as is the case in summer. The air temperature at the time of the winter 

 rains is relatively high, but much lower than in summer. There are also 

 occasional periods of really high temperatures and drying winds. The 

 winter plants, so far as I have observed, usually do not grow as quickly 

 and do not have as large leaf surface as the summer annuals. 



Given a sufficient amount of water both in winter and in summer, it is 

 possible that the difference in the relative temperature relations of soil and 



