278 1JOTANY PART i 



arc often referred to as COMPASS PLANTS. A vertical jtosition is also attained hy 

 tli<' development of phyllodes, in connection with which may 1.x- mentioned the 

 vtTtically placed leaves of many Myrtaceae and Proteaceae ( 82 ). 



The heliotropic character of organs may change through the 

 activity of external influences, and also at different stages of their 

 development and growth. The flower-stalks of JJnin-in cymbaiaria 

 are at first positively heliotropic. After pollination, however, they 

 become negatively heliotropic, and as they elongate they push their 

 fruits into the crevices of the walls and rocks on which the plant 

 ^rnws (p. 257). The intensity of the illumination has a great influence, 

 since plants which in subdued light are positively heliotropic exhibit 

 negative heliotropism when the illumination is excessive. Between 

 the two reactions a neutral aheliotropic condition exists. OLTMANNS. 

 who elucidated this variation of the reaction with the intensity of 

 the light, and the search for an optimal intensity which is connected 

 with it (cf. heliotactic swarm-spores, p. 265), termed it PHOTOMETRY. 

 It remains to be noted that it is not so much the absolute intensity 

 of the light which is of importance, but that the degree of change in 

 light intensity needed to alter the reaction of the plant depends on 

 the illumination to which the latter has been previously exposed ( 83 ). 



Heliotropic sensibility is markedly increased when traces of coal- 

 gas, etc. are present as an impurity in the atmosphere. This is so 

 strikingly the case that conclusions as to' the degree of impurity can 

 be drawn from the heliotropic deflection exhibited by susceptible 

 plants (Peas, Vicia calcarata, etc.). 



B. Geotropism 



That the stems of trees and other plants should grow upwards and 

 their roots downwards, is such a familiar occurrence and so necessary 

 for the performance of their respective functions as to seem almost a 

 matter of course. Just as in the discovery of gravitation 240 years 

 ago, it required an especially keen spirit of inquiry to lead to the in- 

 vestigation of this everyday phenomenon. The fact that everywhere 

 on the earth, stems take a perpendicular direction ; and that, while 

 buried in the earth, this same direction is assumed with certainty by 

 germinating seeds and growing shoots ; and chiefly the fact also that 

 a shoot, when forced out of its upright position, curves energetically 

 until it is again perpendicular, led to the supposition that the cause 

 of these phenomena must be in a directive force proceeding from the 

 earth itself. The correspondence in the " behaviour of a stem in 

 always assuming a perpendicular position, with the continued 

 maintenance of the same direction by a plumb-line, suggested at once 

 the force of gravitation, and the English investigator KNIGHT, in 

 1809, demonstrated that the attraction of gravitation, in fact, exerted 

 an influence upon the direction of growth. As KNIGHT was not able 



