Winifred E, Brenchley 239 



the production of the rootlet. In this way a great increase in absoiption 

 can be provided for with coiii])uratively little increase in the dry weight 

 of the root. The same structure is also the most economical for the 

 performance of the second function of fixation, as the multitude of fine 

 strands result in a very effective su})porting agent. The shoot, on the 

 other hand, is chiefly concerned with the processes of respiration, 

 transpiration and assimilation. For these a considerable bulk of tissue 

 is desirable in order to provide an adequate supply of intercellular spaces, 

 chlorophyll and storage tissue and also that a broad surface may be 

 presented to make the most effective use of sunlight in the elaboration 

 of food material. In order to carry a great expanse of leaf stout stems 

 are necessary, and consequently a large amount of material has to be 

 used in the construction of the shoot. As the plant gets older, the root 

 is increasing its absorbing capacity at the expense of comparatively 

 little new material while the shoot is using up a great deal for growth and 

 may also be acting as a storehouse of reserves, so the relative weights of the 

 two parts tend to diverge more and more. The more rapid the growth, 

 the more rapid the divergence, as is shown graphically by the large 

 shoot/root ratio attained under optimum conditions. 



The behaviour of root and shoot with regard to temperature is 

 distinctly different. It has already been shown that during the first 

 period of growth, whatever the season and the temperature, the weight 

 of the shoot falls and then rises again, whereas the root increases 

 steadily in weight from the beginning. After this period the tempera- 

 ture factor is very potent. The shoot weight at all seasons continues 

 to rise, slowly with low mean maximum temperatures, rapidly with 

 high mean maxima. Under no circumstances with the mean maxima 

 attained in this experiment did the increase in dry weight of the shoot 

 cease even for a time until the very end when growth was completed. 

 Up to a certain limit, also, increase in the rate of shoot growth is corre- 

 lated with rise of temperature. Root growth, on the other hand, is much 

 affected by low mean maximum temperatures, and practically no advance 

 was made in any series, irrespective of the food supply, from mid- 

 November to January, over a period during which the mean maxima 

 were consistently below 60° F. Rise in temperature does not have the 

 same relative beneficial action on the root that it does on the shoot, as 

 no sharp rises in root weight could he detected when temperatures went 

 up. There is some indication that whereas the roots can grow satisfactorily 

 in high temperatures when they are subjected to plenty of heat from 

 an early period of life, root growth is severely checked or even stopped 

 if a sudden and prolonged increase in temperature occurs when the plant 



