352 BOTANY PART i 



from the tip of the cotyledon, is capable of curvature. In this case there is a well- 

 marked distinction between a perceptive organ and a motile organ ; the similarity 

 to corresponding phenomena in geotropism and in the animal kingdom is very 

 striking. There is an essential difference, however, in the method of transmission 

 of the stimulus; "Nerves" are completely wanting in the' plant, and the 

 stimulus is conveyed from cell to cell ( 109 ). 



There is no doubt that the perception of light by the plant is closely connected 

 with photochemical processes. As to how the plant perceives the direction of the 

 light we are, however, ignorant ( 109 ). 



3. CHEMOTROPISM ( no ) 



In the same way as light and gravity, heat and electricity, when 

 their action is one-sided, may bring about directive movements of the 

 plant. Since, however, these movements play no great part in nature 

 they need not be further considered. Those directive movements 

 which are brought about by the unequal distribution of dissolved or 

 gaseous substances in the neighbourhood of the plant are of much 

 greater importance ; these movements are termed chemotropic. 



In the case of fungi and of pollen-tubes, chemotropic movements 

 have been demonstrated which bring the organism into a certain 

 concentration of particular substances ; this concentration is the 

 optimal one. With the same organism and the same stimulating 

 substance these movements are sometimes positive and sometimes 

 negative ; positive when they lead towards a higher concentration of 

 the substance, and negative in the converse case. In the case of 

 pollen-tubes sugar is the chief substance that acts as a stimulus ; 

 in fungi, in addition to sugar, peptone, asparagin, compounds of 

 ammonia and phosphates. There are also substances such as free 

 acids which always have a repellent influence even in extremely weak 

 concentration. Chemotropic irritability has also been demonstrated 

 in roots, though it cannot be said that it plays an important role in 

 their life. 



In the examples of chemotropism given above, the stimulating 

 substances were solid substances in solution. When on the other 

 hand the plant is induced to perform directive movements by the 

 unequal distribution in a space of aqueous vapour or gases, a distinct 

 name has been required, though no distinction of principle can be 

 drawn. Irritable movements caused by differences in moisture are 

 termed HYDROTROPIC, while those brought about by gaseous differences 

 are termed AEROTROPIC. Aerotropism has been proved for pollen 

 tubes, roots, and shoots, and hydrotropism for roots and moulds. 

 Thus roots are positively hydrotropic and seek out the damper spots 

 in the soil by reason of this irritability. The sporangiophores of the 

 Mucorineae are negatively hydrotropic and thus grow out from the 

 substratum. These reactions may be so energetic as to overcome other 

 (e.g. geotropic) stimuli. 



