156 Practical Plant Biology. 



water, nitrates and other mineral substances from the soil, and 

 partly by the mesophyll, carbon dioxide from the air. The wide 

 separation of these parts necessitates a very perfect system of trans- 

 port. This is provided in the conducting tracts or vascular 

 bundles. The motive force to effect this transport is the heat 

 energy absorbed by the leaf-cells. Energy absorbed by the meso- 

 phyll cells, either in the form of light or heat, causes evaporation 

 from their surfaces into the air-spaces of the leaf. This loss tends 

 to diminish the bulk of the watery contents of the vacuoles of the 

 cells. But the osmotic pressure of the dissolved substances in the 

 vacuoles opposes the collapse of the protoplasm and presses it 

 outwards, so water has to be drawn into the vacuole. A supply 

 for this is at hand in the cells of the sheath which in turn draw 

 water from the woody tracheids. These latter afford an easy 

 passage for water rising from the roots through the stem and leaves. 

 Thus the evaporation from the mesophyll cells draws the water 

 up through the capillary tubes of the wood of the conducting 

 tracts from the root, where the supply is kept up by the absorp- 

 tion of the root-hairs. 



Evidently in a low-growing plant like the fern if evaporation re- 

 moves water from the upper end of the tracheidal tubes the atmos- 

 pheric pressure acting on the water surfaces round the root-hairs 

 would force up water to take the place of that which is lost by 

 evaporation. But in plants much higher than 30 feet, which 

 is near the limit to which atmospheric pressure can raise water, 

 evidently such a mechanism must break down. As a matter of 

 fact even low-growing plants need not rely on atmospheric pressure 

 for the lifting of the water in their tracheidal tubes. For water, when 

 contained in vessels which it thoroughly wets (as in the tracheids) 

 and completely fills, leaving no bubbles, transmits a pull just in 

 the same way as a metal wire would. Thus the removal of water 

 from the mesophyll cells by evaporation produces a pull in the 

 water of the vacuoles ; this is transmitted to, and down through, 

 the water contained in the tracheidal tubes throughout the con- 

 ducting tracts of the plant. The mesophyll cells are pre- 

 vented from collapsing under this stress by the osmotic pressure 

 of the dissolved substances in their vacuoles, while the ladder-like 

 and spiral thickenings of tracheids support their thin walls and 

 prevent them buckling inwards. Evidently the pits are essential 

 in order that the flow of water may be facilitated from one 

 tracheid to another. 



The elimination of water vapour from the mesophyll cells 

 through the passages and stomata is called transpiration and the 



