248 BOTANY 



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 accom- 

 plished, as, for example, the dehiscence of seed-vessels and the dis- 

 semination and burial of seeds. 



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

 special apertures (Pa/paver, 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 distance ( Tricoccae, etc. ). In certain fruits not only curvatures but torsions 

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

 Erodium gruinum (Fig. 200), Stipa pennata, Avena sterilis, by means of which, in 

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



The opening and closing of the involucre of many Oompositae (Erigeron, Carlina, 

 etc.) at the time of the ripening of the seeds, and the changes in the position of the 

 pappus-hairs (Taraxacum, Tragopogon, etc.), are also due to hygroscopic movements 

 resulting from variations in the amount of moisture in the atmosphere. In dry 

 weather the pappus is spread out in the form of an umbrella, but in wet weather it 

 closes up. The opening of anthers and sporangia, the rupture of moss-capsules, 

 and the dissemination of the spores of the JSquisetaceae, Sepaticae, and Myxomyceles 

 are also effected by similar movements. Anthers and sporangia possess peculiarly 

 thickened cells (fibrous cells, annulus), by the contraction of which their dehiscence 

 is produced. The opening of the moss-sporangium is, in like manner, due to the 

 hygroscopic movements of the teeth of the peristome, while the sporangia of the 

 Liverworts are provided with specially thickened spiral bands (elaters), which, like 

 the capillitia of the Myxomycetes, effect the discbarge of the spores. In the case 

 of the Equisetaceae the outer walls of the spores themselves (the perinium) take 

 the form of four arms, which, like elaters, are capable of active movements, by 

 means of which numbers of spores become massed together before germinating, and 

 the isolation of the dioecious prothallia prevented. 



In order to call forth imbibition movements the actual presence of liquid water 

 is not necessary ; for, through their hygroscopicity, cell walls have the power of 

 absorbing moisture from the air. They are hygroscopic, and for this reason the 

 ensuing movements are also often termed hygroscopic movements. 



2. Growth Curvatures 



Movements from which curvatures result are, for the most part, 

 produced by the unequal growth of living organs. The unequal 

 growth is due, partly to internal causes which are still undetermined, 

 and partly to the operation of external influences which can be posi- 

 tively demonstrated and defined. The movements resulting in the 

 first case are spontaneous, and are called autonomic movements or 

 NUTATIONS; in the second case the movements are the result of 

 external stimuli, and are distinguished as irritable or PAEATONIC 



MOVEMENTS. 



Autonomic Movements are most plainly apparent in young 

 actively-growing organs, although nutations have been shown to be 



