2o8 THE BIOLOGY OF FLOWERING PLANTS 



and the figures show that the leaf has an ash content on the 

 average double that of the stem when both are mature. 

 In young plants, on the other hand, the ash content of the 

 stem is slightly greater. We have here a distinct indication 

 of a concentration of ash in the transpiring leaf. The 

 xerophytic leaf of the conifers has a very much lower ash 

 content than that of mesophytes in general. Taking 

 conventional figures, we may put the one at i'5 per cent, of 

 dry weight, the other at lo per cent. Moreover, the ash 

 content of the pine needle mounts with the years of its age. 

 For the young leaf of Pinus austriaca it is i'6 per cent., at a 

 year old i"8 per cent., at 2 years 2*7 per cent., at 3 years 

 3-17 per cent., and at 4 years 4*55 per cent. ; on the other 

 hand, shade leaves are said to be richer in ash than sun 

 leaves. Huber (1923) finds that the basal shoots of Sequoia 

 transpire more vigorously than the apical, and that they 

 have also a higher ash content. McLean (191 9) found 

 the ash content of the leaves of sun plants to be lower 

 than that of the leaves of shade plants of the forests of 

 Brazil. Such figures are suggestive, but even if they were 

 unambiguous they would not be conclusive. 



It might be thought that the relation of transpiration to 

 mineral supply could be easily tested experimentally, but 

 the difficulty of maintaining all conditions except tran- 

 spiration rate equal over a long period is very great. Various 

 investigations have been made, but without yielding a 

 satisfactory answer. Hasselbring (1914) grew two sets 

 of tobacco plants side by side, shading one set from the sun 

 by gauze. The two sets attained exactly the same dry 

 weight ; the ash of the shade set was ii'2 per cent, com- 

 pared with 9'6 per cent, in the sunset. The shade plants 

 had absorbed 35 htres of water, and the sun plants 46 litres, 

 or 30 per cent. more. The amount of minerals absorbed 

 was therefore not proportional to transpiration, nor did the 

 plants with lower transpiration suffer at all in this respect. 

 But in this experiment other conditions besides evaporation 

 were different. More important is the fact that tran- 

 spiration (deduced from the amount of water absorbed) 



