86 , BOTANY OF THE LIVING PLANT 



the amount of water present, which covers each solid particle of which 

 the soil is composed. An important point is that the water forming 

 these films is mobile. It can move from place to place, equalising 

 the thickness of the films throughout, so that if water be drawn off 

 at any point in the soil it can be made good by the flow of compen- 

 sating water towards that point. Consequently any rootlet closely 

 related to the particles of the soil has at its disposal, within certain 

 limits, the whole reservoir of water which the soil in its near neighbour- 

 hood contains. (See Fig. 57.) 



The root-hairs partially enwrap the particles of soil with their 

 gummy walls, and so gain access to the film of water covering them. 

 Each hair is a vacuolated cell, with a film of living cytoplasm inter- 

 vening between the vacuole-fluid within and the covering cell-wall. The 

 latter is a permeable membrane, through which water and substances 

 in solution in it can pass by filtration ; and it has access to the water 

 in the soil, and is saturated by it. But the living cytoplasm is a semi- 

 permeable membrane (p. 25). During life it can control the passage 

 of materials dissolved in its cell-sap, such as sugars. Thus, osmotically 

 active substances are within the protoplasmic film, while outside is 

 the soil-water with various other substances in solution in it. The 

 living protoplasm acting as a controlling influence, there is a passage 

 of water, and also of certain of the substances in solution in it, into 

 the root-hair, while the living protoplasm exercises some degree of 

 selection upon these. Thus, under protoplasmic control, water and 

 certain salts enter into the plant-body. 



The water thus absorbed from the soil is transferred by a " slow " 

 movement from cell to cell. It passes, under the control of the proto- 

 plasts of the cells of the exodermis, into the cortex, which serves the 

 young root as a temporary reservoir. Again, under control of the 

 protoplasts of the endodermis, it passes into the stele, and there meets 

 at near hand the conducting tissue of the xylem. Thence it may be 

 transmitted by a " quick " movement throughout the conducting 

 system of the plant. Finally, it may be distributed again through 

 the agency of individual cells to the points where it is required. 



At the beginning and at the end of the course thus traversed, the 

 Transpiration-Stream passes cell by cell, and its movement is under 

 protoplasmic control. But in the conducting tract of the wood it passes 

 through the cavities of the vessels and tracheides, and through the perme- 

 able pit-membranes. This can be shown by means of coloured fluids, 

 which readily traverse the wood of a living plant from a cut surface, 

 following the cavities of the vessels, which they stain. Negative 



