270 HOTANY I..VKT i 



length of the opposite sides, resulting from unequal TURGOR TENSION. 

 A third source of curvature is found in the unequal amount of water 

 taken up by IMBIBITION, and the consequent unequal swelling of the 

 cell walls on the opposite sides of an organ. A fourth cause is the 

 loss of water from certain cell-complexes, the latter being made to 

 contract in a definite direction owing to the cohesive force of the 

 water which remains in them. 



When the direction of the curvature is determined by the position 

 of the source of the stimulus causing it, the movement is termed tropic 

 (tropism) ; when no such relation is evident and the movement results 

 from the internal disposition of the structure, it is spoken of as nastic. 



1. Movements dependent on Imbibition and Cohesion 



As the cell Avails of actively living cells are always completely 

 saturated with imbibition water, hygroscopic curvatures are exhibited 

 only by dry or drying and, for the most part, dead tissues. The 

 hygroscopic movements in any case, however, are due to the physical 

 properties of the cell walls, and have no direct connection with the 

 vital processes, except in so far as the capacity of cell walls to swell 

 and take up large quantities of imbibition water is due to the proto- 

 plasm by which they were formed. The activity of the protoplasm 

 in the formation of the cell walls is likewise manifested in their 

 anatomical structure, in their stratification and striation, and in the 

 position of the pits. 



The absorption of imbibition water by cell walls is accompanied 

 by an increase in their volume, and conversely the volume of the cell 

 walls is diminished by the evaporation of the imbibition water. Ac- 

 cordingly, whenever unequal amounts of water are held by the cell 

 walls on the different sides of an organ, either through unequal 

 absorption or evaporation, hygroscopic movements are produced, 

 which result in the curvature of the organs. In many cases the 

 organs of plants are especially adapted to such movements, by means 

 of which, in fact, important operations are often accomplished, as, for 

 example, the dehiscence of seed-vessels and the dissemination and 

 burial of seeds. Similar movements also occur, without special 

 purpose, however, as for example in dead branches of Conifers. 



The rupture of ripe seed-vessels, as well as their dehiscence by the opening of 

 special apertures (Papaver, Lychnis, Antirrhinum, etc.), is a consequence of the 

 unequal contraction of the cell walls due to desiccation. At the same time, through 

 the sudden relaxation of the tension, the seeds are often shot out to a great dist.mn' 

 (Tricoccae, Geranium, etc.). In certain fruits not only curvatures but torsions ;m- 

 produced as the result of changes in the amount of water they contain, e.g. Erodiutn 

 gruinum (Fig. 222), Stipa pennata, A vena sterilis ; by means of these, in conjunc- 

 tion with their stiff barb-like hairs, the seeds bury themselves in the earth. 



