334 



BOTANY 



PART 



amount of water they contain, e.g. Erodium gruinum (Fig. 275), Stipa pennata, 

 Avena sterilis ; by means of these, in conjunction with their stiff barb-like hairs, 

 the seeds bury themselves in the earth. 



The opening or closing of the moss sporogonium is, in like manner, due to the 

 hygroscopic movements of the teeth of the peristome surrounding the mouth of 

 the capsule. In the case of the Equisetaceae the outer walls of the spores them- 

 selves take the form of four arms, which, 

 like elaters, are capable of active move- 

 ments. 



In order to call forth imbibition move- 

 ments the actual presence of liquid water 

 is not necessary, for the cell walls have the 

 power of absorbing moisture from the air. 

 They are hygroscopic, and are used to 

 estimate the humidity of the air in hygro- 

 meters and weather-glasses. 



The mechanisms which depend 

 on the cohesive power of water are 

 distinguished from those depending 

 on imbibition. The COHESION MECHAN- 

 ISMS were previously confounded with 

 the latter, from which they differ in 

 that, even during the movement, the 

 cell walls remain saturated with 

 water. It is the lumen of the cell 

 which diminishes in size when the 

 loss of water, on which the move- 

 ment depends, occurs. A good ex- 

 ample is afforded by the movements 

 of the sporangium of the Polypo- 

 diaceae on drying. The sporangia 

 are stalked, biconvex bodies contain- 

 ing the spores within a wall composed 

 of one layer of cells. While the rest 

 of the wall is composed of thin 

 walled cells, one row of peculiarly thickened cells forms a vertically 

 placed semicircle (Fig. 276 R). The cells of this ANNULUS have their 

 outer walls thin, the lateral walls increasingly thickened from the 

 outside inwards, and the inner walls thick. On exposure to dry air 

 the cells of the annulus gradually lose the contained water. The 

 watery contents do not, however, separate from the cell wall nor 

 does a rupture occur in the liquid, since the adhesion to the wall and 

 the cohesion of the molecules of water is very great, amounting 

 to hundreds of atmospheres ( 96 ). A deformation of the cell wall, 

 therefore, follows the diminishing water- content ; the thin outer 

 wall (Fig. 276, 3) is pulled inwards, thus approximating the thickened 

 lateral walls. There thus comes about an energetic one-sided shortening 



FIG. 275. Partial fruit of Erodium gruinum. 

 A, in the dry condition, coiled ; B, moist 

 and elongated. (After NOLL.) 



