298 PLANT STEUCTUEES 



attract to further study. It is merely the opening of a door 

 to catch a fleeting glimpse. 



A common division of the subject presents it under five 

 heads : (1) Stability of form, (2) Xutrition, (3) Eespira- 

 tion, (4) Movement, (5) Reproduction. 



STABILITY OF FOEM 



157. Tvtrgidity. — It is a remarkable fact that plants and 

 parts of plants composed entirely of cells vi^ith very thin and 

 delicate walls are rigid enough to maintain their form. 

 It has already been noted (see § 30) that such active cells 

 exert an internal pressure upon their walls. This seems to 

 be due to the active absorption of liquid, which causes the 

 very elastic walls to stretch, as in the "blowing up " of a 

 bladder. In this way each gorged and distended cell be- 

 comes comparatively rigid, and the mass of cells retains its 

 form. It seems evident that the active protoplasm greedily 

 pulls liquid through the wall and does not let it escape so 

 easily. If for any reason the protoplasm of a gorged cell 

 loses its hold upon the contained liquid the cell collapses. 



158. Tension of tissues. — The rigidity which comes to 

 active parenchyma cells through their turgidity is increased 

 by the tensions developed by adjacent tissues. For exam- 

 ple, the internal and external tissues of a stem are apt to 

 increase in volume at different rates ; the faster will pull 

 upon the slower, and the slower will resist, and thus be- 

 tween the two a tension is developed which helps to keep 

 them rigid. This is strikingly shown by splitting a dande- 

 lion stem, when the inner tissue, relieved somewhat from 

 the resistance of the outer, elongates and causes the strip 

 to become strongly curved outward or even coiled. Experi- 

 ments with strips from active twigs, including the pith, 

 will usually demonstrate the same curve outward. Tension 

 of tissues is chiefly developed, of course, where elongation 

 is taking place. 



