300 BOTANY tart i 



for life, and the organs of plants first assume such positions in air, or 

 water, or in the earth as are necessary for the performance of their 

 vital functions. A green plant which spread its roots over the surface 

 and unfolded its leaves below ground could not exist, even though all 

 its members possessed the best anatomical structure. Seeds are not 

 always deposited in the soil with the embryonal stem directed upwards 

 and the radicle downwards, so that their difterent organs can, merely 

 by direct growth, attain at once their proper position. A gardener 

 does not take the trouble to ascertain, in sowing seed, if the end which 

 produces the root is directed downwards or the stem end upwards ; 

 he knows that in any position the roots grow into the ground and the 

 stems push themselves above the surface. Plants must accordingly 

 have in themselves the power of placing their organs in the positions 

 best adapted to the conditions of their environment. This is only 

 possible, however, when the externally operative forces and substances 

 can so influence the growth of a plant that it is constrained to take 

 certain definite directions. 



The same external influences excite different organisms or different 

 parts of a plant, and even the same structure in different stages of 

 its development, to assume different positions. Thus, for instance, the 

 young Plasmodia of Myxomycetes avoid the light, while swarm-spores 

 of Algae move towards the light. The motile spheres of Volvox seek 

 a less intensity of light when they contain egg-cells than when they 

 are in a purely vegetative stage. The stem grows towards the light, 

 the root away from the light ; both grow in the direction of the rays 

 of light. The leaves on the other hand place themselves with their 

 surfaces at right angles to the rays of light falling on them. 



These examples only indicate a few of the modes of reaction of the 

 organism. In the particular description of the stimulus movements 

 it will always be necessary to make clear what is the determining 

 stimulus. Light, heat, gravity, contact, material and mechanical 

 influences of the most varied kind can serve as stimuli. It has to 

 be carefully determined whether the movement is one of locomotion or 

 merely a curvature ; the former is termed taxis, the latter tropism or 

 nastic movement. We speak of tropism when the organ takes up a 

 resting position definitely related to the direction of the effective 

 stimulus. Nastic movements on the other hand are curvatures which 

 bi-ing about a particular position in relation to the plant and not to 

 the direction of the stimulus. 



In the following account the tactic and tropic movements which 

 lead to a definite orientation of the plant in space Avill be grouped 

 toiiethcr. The other movements will be termed nastic. 



■'o^ 



The condition of reccptivcnoss to stimuli in the ]ilant is common to all irritable 

 movements and indeed all irritable phenomena. The stimulus must give rise to 

 definite changes in the plant, the protoplasm must react to these changes in such a 

 way that the characteristic externally visible reaction ultimately takes'place. Between 



