THE WATER-RELATION 85 



The Shuut and Root together form this WLiter-cuntainiiii^ PkiiiL- 

 Body. Their eonstituent tissues are continuous from one to thu 

 other. This is especially to be recognised in the case of the vascular 

 skeleton. It may be traced as extending without break from the apex 

 of each root continuously to the main root, and onwards to the base of 

 the stem, where its strands undergo a rearrangement in passing from 

 root to stem, but no break of continuity. Thence the strands of the 

 vascular system extend upwards to the leaf-bases, and out to their 

 extreme tips and margins. Similarly with the axillary buds, or the 

 branches developed from them : their vascular system is connected 

 at the base with that of the main shoot. This continuous vascular 

 skeleton is embedded in softer tissues, mostly parenchymatous, anrl 

 with a relatively high water-content. The dermal tissue forms a 

 complete superficial skin over the whole shoot. It covers the surface 

 of stem and leaf, and is even extended downwards, though with modi- 

 fication of its characters, to the root. Since its cells form a continuous 

 skin, and their outer walls are covered by an impervious film of cuticle, 

 the shoot is thereby protected from indiscriminate loss by surface- 

 evaporation of the water it contains in so large a quantity. But as 

 the atmosphere in which the shoot is exposed is almost always below 

 the point of saturation with water-vapour, such evaporation is liable 

 constantly to occur, just as in clothes hung out to dry. It is checked, 

 however, by ttie cuticular covering, except where this is interrupted by 

 the pores of the stomata. These are present on the stem, but arc 

 specially numerous on the lower surface of the leaves. Thus the 

 superficial skin is porous. The storaata are not always open ; they act 

 automatically, opening and closing according to external conditions. 

 Thus they serve as an adjustable control upon the loss by evaporation, 

 or, as it is termed, Transpiration. The loss is made up in the normal 

 plant by absorption of water from the soil by the roots. It is then 

 transmitted upwards along the vascular tracts. Thus a stream of 

 water passes through the plant, varying, in volume according to the 

 conditions. This is called the Transpiration-Styeani. 



The Transpiration Stream 

 The root-system from which this stream originates is buried in 

 the soil, which is a mass of mixed materials, organic and inorganic, 

 holding within it water in greater or less degree. No soil in the open 

 is ever actually dry. Normally the amount of water it contains is 

 considerable. It is partly held as water of imbibition in the Humus. 

 Partly it consists of a film of water, of varying thif'kness according to 



