312 BOTANY 



PART I 



While pbototropism is increased by impure air (p. 306), geotropism 

 is, as a rule, weakened hy this influence. 



The discoverer of geotropism, Knight, attemjDted to explain the 

 geotropic movements on purely mechanical lines ; this did not seem 

 difficult, especially for positively geotropic organs. He regarded them 

 as simply following the attractive force of gravity till a condition of 

 rest is attained. Later Hofmeister advanced similar views. The 

 correct assumption that we are concerned with complicated stimulus 

 mechanisms in which terrestrial gravity only plays the part of the 

 liberating factor depends on the work especially of Dutrochet, 

 Frank, and Sachs. Even the one fact that the root can carry out 

 its geotropic curvature against the resistance of mercury is sufficient 

 to call in question every purely mechanical explanation. 



Only in recent times has the attempt been made to determine 

 what is the primary eftect of gravity in the plant (i^"a^. E. Noll 

 first elaborated the idea that there must be some bodies in the cells 

 of greater specific gravity than the surrounding protoplasm, and 

 capable iinder the influence of gravity of exerting a one-sided pressure 

 on the protoplasm ; on this taking place the protoplasm directs the 

 processes of growth in accordance with the direction of the force of 

 gravity. Nemec and Haberlandt then suggested that these specific- 

 ally heavier bodies might be found in certain starch grains which show 

 relatively rapid movements of falling in the cells. They foimd such starch 

 grains in the pericycle of the stem (p. 129) and in the cells of the root- 

 cap. They assume that the stimulus of gravity can only affect directly 

 portions of the plant provided with such starch grains, but that it may 

 be conducted from these points to others. As a matter of fact the 

 attempt had previously been made to show that only the tip of the 

 root can receive the gravitational stimulus. Even at the present time 

 this question is not decided, and not a few investigators assume that 

 all cells — though in various degrees — are geotropically sensitive. The 

 hypothesis of NfiMEC and Habkrlandt, though there is much in its 

 favour, is not fully established. 



Twining" Plants (^^'^), which are found in the most various families 

 of plants, have shoots which require to grow erect but are unable to 

 support their own weight. The erect stems of other plants, which 

 often secure their own rigidity only by great expenditure of assimi- 

 lated material (in xylem and sclerenchyma), are made use of by 

 stem-climbers as supports, on which to spread out their assimilatory 

 organs in the free air and light. The utilisation of a support pro- 

 duced by the assimilatory activity of other plants is a peculiarity they 

 possess in common with other climbers, such as tendril- and root- 

 climbers. Unlike them, however, the stem-climbers accomplish their 

 purpose, not 1)y the help of lateral clinging organs, but by the 

 capacity of their main stems to twine about a support. The first 

 internodes of young stem-climbers, as developed from the subterranean 



