154 PHYSIOLOGY OF NUTRITION 



bearing cells of a leaf during a period of sunlight, diffuses as a solute, mainly to the 

 periphery of a sub-stomatal gas space, where it passes out of solution and then diffuses 

 as a gas through the stomatal pore into the surrounding atmosphere. Of course it 

 may also diffuse in other directions through the tissues. The gas spaces of the xylem 

 are not continuous with those of the cortex, but gases may move from one system of 

 channels to the other, first passing into solution, then diffusing as solutes, and finally 

 passing out of solution again. These gas spaces of the xylem are generally not inter- 

 cellular; they occupy portions of the vascular channels (that is, the interiors of much 

 elongated cells that are dead and without protoplasm and that have lost their end 

 walls in many cases where adjacent cells were originally in contact). The pressure of 

 the gas in the vessels is frequently much lower (especially when the transpiration rate 

 is high) than that of the environmental atmosphere and of the intercellular cortical 

 channels. On the other hand, the gas pressure in the xylem may sometimes be higher 

 than that in the cortical channels (when there is sap pressure). 



3. Movement of Water and Dissolved Substances. — Girdling experiments show 

 (1) that water and soil solutes move from the roots to other parts of the plant body 

 through the xylem vessels that are not blocked with gas; and (2) that organic solutes 

 (such as sugar) move from the leaves, and from regions where such substances have 

 been accumulated, to other regions, through the phloem of the vascular tissue. 



4. The Transpiration Stream. — Water evaporates from the water-impregnated 

 cell walls that bound the sub-stomatal gas spaces of leaves, and it then diffuses, as 

 water vapor, through the stomatal openings into the external atmosphere. This 

 process is called stomatal transpiration. Water also evaporates directly into the 

 atmosphere, but at a slower rate, from the cutinized epidermal cell walls which always 

 contain some imbibed water. This process is cuticular transpiration. Transpiration 

 tends to dry the cell walls from which the water evaporates, and thus to increase the 

 imbibitional attraction they exert on the liquid water within the cells and farther back 

 in the tissues. Equilibrium tends to be reestablished by movement of water out of 

 the xylem vessels, through intervening cells, to the evaporating surfaces. The forces 

 drawing water out of the vessels are very great, and they tend to stretch the whole 

 water mass of the plant body. The vessels are sufficiently rigid to prevent their being 

 collapsed by this inward pull on their walls, and the strain (by virtue of the cohesion of 

 water) is transmitted to all parts, tending to remove some water from all cell walls 

 whose outer surfaces are in contact with gas. At root surfaces this tendency results 

 in the drawing in of water from the soil (probably carrying dissolved substances with it, 

 in so far as the cell membranes are permeable to these solutes). As transpiration pro- 

 ceeds, so long as the water supply is maintained at the absorbing root surfaces (espe- 

 cially root-hairs), there is a flow of water into the roots, through the xylem vessels, and 

 into the cell walls from which evaporation is occurring. This mass flow of water 

 (carrying solutes) through the plant is called the transpiration stream. Some of the 

 water drawn into the roots is used in tissue enlargement (which is primarily imbibi- 

 tional and osmotic swelling), in the photosynthesis of carbohydrates, etc., and con- 

 sequently the rate of water absorption by the root' system is, on the whole, for long 

 periods, a little greater than the rate of transpiration. Also, some water is lost, in 

 some plants, by being excreted to the exterior in the liquid form, as from hydathodes 

 and nectaries, which excrete aqueous solution at leaf margins, on flower parts, etc. 

 This glandular excretion of aqueous solution by hydathodes is termed guttation. 

 Compared with transpiration, guttation is a slow and not very important process; 

 it is encountered in comparatively few plants and is not maintained for long periods. 



