148 THE VICTORIAN NATURALIST. [Vol. XXIII. 



humus soils. The aeration of the soil is also fully discussed, for 

 Grew considered that plants absorbed air mainly by means of 

 their roots. 



The first recorded experiments upon geotropism were per- 

 formed by Grew (p. 136), and these clearly showed that the 

 downward growth of the root is not merely due to a tendency to 

 grow into the soil (somatotropic or chemotropic attraction), for 

 the curvature occurs equally well when the root is reversed but 

 surrounded on all sides by soil. The tendency of the stem to 

 grow upwards was considered to be due to a magnetic correspon- 

 dence between the air-vessels and the atmosphere. Grew 

 evidently sought an analogy with the compass needle, plants 

 being credited with a tendency to place their long axes parallel 

 to the supposed magnetic lines of force radiating from the air. 

 It was, however, noticed that certain stems and roots grow 

 horizontally, and that the roots of many plants annually pull the 

 stem deeper into the ground. The latter is an observation which 

 has recently been shown by Rimbach to apply to a large number 

 of " geophilous plants." It was reserved for Knight to rectify 

 Grew's misconception, and to show that gravity is the direct 

 exciting cause of the downward curvature of primary roots and 

 of the upward growth of seedling stems, and that it may be 

 replaced by centrifugal force. 



The remarks upon the winding of stems, tendrils, &c., are 

 very vague, and largely incorrect, although they seem to indicate 

 that the phenomenon of circumnutation had already been 

 observed. It is, however, possible that in stating that " the 

 claspers of the vine have a motion of convolution " he may 

 merely refer to their tendency to become spirally coiled (p. 137). 



Grew's ideas upon constructive and nutritive metabolism seem 

 to us highly grotesque, but, considering the almost entire absence 

 of any accurate chemical knowledge, and the absurd speculations 

 then rife, nothing more could be expected, and although the basis 

 is unsound the conclusions are usually logically correct, even if 

 erroneous according to the knowledge of the present day. 

 Sulphur played a great part in the chemistry of the time, and 

 when Grew observed that the carbonized residue of a plant 

 largely disappeared, leaving a certain amount of ash when further 

 heated, he supposed that this was due to the " sulphur " as it 

 volatilized carrying off with it the greater part of the plant's sub- 

 stance, and the same reason is given for the almost entire 

 absence of any residual ash when starch is burnt (p. 258). Grew's 

 observations (p. 261) upon the crystalline salts which may be 

 extracted from the residual ash of plants led him to conclude 

 that the different tissue elements were formed by the deposition 

 and apposition of crystalline elements, so that we have here 

 apparently the germ of Nageli's micellar theory. Grew, however, 



