150 



VEGETABLE LIFE AND WORK. [SECTION 16. 



transparent aquatic plants aad in hairs ou the surface of land plants (where 

 it is easiest to observe), that it may be inferred to take place in all cells 

 during the most active i)art of their life. This motion is commonly a 

 streaming movement of threads of protoplasm, carrying 

 along solid granules by which the action may be ob- 

 served and the rate measured, or in some cases it is a 

 rotation of the whole protoplasmic contents of the cell. 

 A comparatively low magnifying power will show it in 

 the cells of Nitella and Cliara (which are cryptogamous 

 plants) ; and under a moderate power it is well seen in 

 the Tape Grass of fresh water, Vallisneria, and in Naias 

 flexilis (Fig. 489). Minute particles and larger green- 

 ish globules arc seen to be carried along, as if in a cur- 

 rent, around the cell, passing up one side, across the 

 end, down the other and across the bottom, completing 

 the circuit sometimes within a minute or less when well 

 warmed. To see it well in the cell, which like a string 

 of beads form the hairs on the stamens of Spidcrwort, 

 a high magnifying power is needed. 



462. Transference of Liquid from Cell to Cell, 



and so from place to place in the plant, the absorption 



of water by the rootlets, and the exhalation of the 



greater part of it from the foliage, — these and similar 



operations are governed by the physical laws which 



regulate the diffusion of fluids, but are controlled by the 



action of living protoplasm. Equally under vital control 



*^^ are the various chemical transformations which attend 



assimilation and growth, and which involve not only molecular movements 



but conveyance. Growth itself, which is the formation and shaping of 



new parts, implies the direction of internal activities to definite ends. 



403. Movements of Organs. The living protoplasm, in all but the 

 lowest grade of plants, is enclosed and to common appearance isolated in 

 separate cells, the walls of which can only in their earliest state be said to 

 be alive. Still plants are able to cause the protoplasm of adjacent cells 

 to act in concert, and by their combined action to effect movements in 

 roots, stems, or leaves, some of them very slow and gradual, some manifest 

 and striking. Such movements are brought about through individually 

 minute changes in the form or tension in the protoplasm of the innumera- 

 ble cells which make up the structure of the organ. Some of the slower 

 movements are effected during growth, and may be explained by inequality 

 of growth on the two sides of the bending organ. But the more rapid 

 changes of position, and some of the slow ones, cannot be so explained. 



Fig. 489. A few cells of a leaf of Naias flexilis, highly maguiiied: the arrows 

 indicate the courses of the circulating currents. 



