412 MANUAL OF BOTANY 



the cell-wall is in contact with water on the outside. The 

 protoplasm is actively living, and in the course of the chemical 

 changes which are incident to vital action, certain substances are 

 produced by it, which, like the syrup in the experiment detailed, 

 have an affinity for water, or, to use a more technical phrase, have 

 a fairly high osmotic equivalent. Water consequently passes 

 into the cell, at first only in such quantities as to somewhat 

 distend it. As the process goes on, more liquid is present than 

 can be stored in the molecular interstices of the protoplasm. 

 Drops consequently appear, and these gradually run together 

 until a distinct though small vacuole, or a number of such 

 vacuoles, is apparent in the protoplasm. By a continuation of 

 the process the protoplasm ultimately forms a layer round the 

 cell-wall, enclosing the large cavity in which the surplus liquid 

 is held. 



But, as has already been said, the process is not a simple 

 physical one. Though the conditions of the first experiment are 

 approximated to, they are not altogether realised. The syrup in 

 the bladder finds its representative in the osmotic substances" 

 formed by the protoplasm and dissolved in the water in its 

 meshes ; the water outside the cell is the same as the water in 

 the outer vessel. But there is a great difference in the membrane. 

 The bladder of the experiment is replaced by a film of cellulose 

 lined by the protoplasm itself. The former is readily permeable, 

 but the latter is not. A further experiment will show a very 

 important modification of the process, brought about by the 

 protoplasm, and demonstrating that the entry of water into the 

 cell is very largely under the regulation of the latter. 



Take a cell of the cortex of the plant and put it in contact 

 with a liquid of higher osmotic power than its own contents ; 

 for instance with a solution of common salt, containing 5 — 10 

 per cent, of the crystals. Watch its action on a slide under the 

 microscope, and let the salt solution be coloured with some 

 vegetable dye which will not injure the living substance. As the 

 salt solution reaches the cell, the protoplasm of the latter 

 gradually retreats from the walls, at first at the corners and then 

 aU round the sides, till it appears as a roundedmass in the centre. 

 The salt solution has abstracted the water from the vacuole, and 

 the protoplasm, relieved of the outward pressure caused by the 

 liquid in the latter, has shrunk away from the walls. The outward 

 stream has been accompanied by an incoming one as in the first 

 experiment. The coloured salt solution will be seen inside the 

 cell-wall, between it and the protoplasm. But here is a great 



