THE LIFE-HISTOKY OF PLANTS. 



215 



and buds are at tto period centres of activity, so the 

 flow is mainly an upward one. In autumn, when 

 consolidation of tissues and storage of nutritive 

 matters are the chief operations of the plant, there is 

 an increased necessity for the presence of matters 

 formed in consequence of leaf -action, and the flow is 

 then, to a large extent, a downward one. But there 

 is no absolute difference between crude and elabo- 

 rated saps, and no absolutely fixed course for them to 

 take. The ascending sap consists of water pumped 

 up from the roots to supply the requirements of the 

 growing cells and the chemical changes going on in 

 them, and the excess of it is evaporated when the 

 leaves expand ; but, though mainly watery, it con- 

 tains some mineral matters, and also some ingre- 

 dients which must have been derived from the 

 action of the leaves of the preceding season." 



In what manner the fluid absorbed by the roots is 

 taken up has been explained ; how it passes as it, or 

 some of it, does, against gravity, from the roots to 

 the leaves of the tallest tree, is still, to a consider, 

 able extent, a matter of controversy. The assigned 

 causes are diffusion, before explained ; capillary at- 

 traction, or the adhesion and upward movement of 

 fluids in tubes of minute bore ; atmospheric pressure, 

 ■or that exercised by the elasticity of the ceU-waUs, 

 \ or by the expansion by heat of tho bubbles of air 

 mixed with the fluid, which in their expansion push 

 the fluid before them ; the swaying to and fro of 

 branches, &c. When so many explanations are 

 given, it is pretty certain that we have not yet 

 ■arrived definitely at the true one. 



Bleeding of "Vines. — In spring, when the in- 

 creasing temperature stimialates the roots to renewed 

 action, the plant becomes gorged with water, because 

 at first there is no transpiration from the leaves to 

 balance the influx, and the air within the tissues 

 liecomes consequently compressed. If now the 

 plant be wounded, the sudden expansion of the air 

 pushes up the fluid, and much of it escapes, as in the 

 so-called " bleeding " or " tears " of the vine. The 

 analogy, however, whether between the ascending 

 sap and the blood, or between it and tears, is almost 

 whoUy fanciful, as the fluid in question has few of 

 the characteristics of those whose names it bears. 

 The a'scending sap consists almost entirely of pure 

 water, with traces of mineral ingredients, and some- 

 times with a notable quantity of sugar. Attfield 

 has lately shown that the tears of the Birch (of 

 which in one day 63 lb. were collected from one 

 tree, and 1,486 lb. from November to May) con- 

 tained 99 per cent, of water, with 1 per cent, of 

 solid matter, eleven-twelfths of which latter con- 

 sisted of sugar-. The sugar had probably been 

 washed out from the cells during the upward rush. 



At any rate, were this so-called ascending sap a 

 highly nutritive fluid, its loss in such large quanti- 

 ties as sometimes happens (as in the collection of 

 Maple-sugar) would be productive of much more 

 serious results than is usually the case, for the fears 

 of gardeners as to the ill-efiects of bleeding in the 

 Vine are, if not whoUy without foundation, at least 

 not proportionate to the observed consequences. 



Tissues thyougli which, the Sap Passes. — 



The so-called ascending sap has been ascertained to 

 pass up through the soft young wood, either thi-ough 

 the cavities of the cells, passing from one to tho other 

 by osmosis, as happens in spring, when the tissues 

 are gorged, or by imbibition through the walls of 

 the cells, mounting thus from cell to cell as the 

 water mounts in a piece of blotting-paper, the lower 

 end of which is soaked in water. 



The fluid contained in a leaf may, therefore, be 

 considered to have been derived from the stem, 

 which, in its turn, acquired it through the agency of 

 the roots. Under certain circumstances, no doubt, 

 leaves, or even stems, may themselves absorb water 

 or watery vapour directly from the atmosphere, as 

 is proved by the effects of syringing after a hot dry 

 day ; but for the present purpose water so obtained 

 may be disi-egarded, and wo may confine ourselves to 

 the juices in the leaf which have been derived more 

 or less circuitously and indirectly from the root. 

 These leaf -juices undergo various changes as a direct 

 consequence of their exposure to the atmosphere. 

 They take in some gases ; they give out others. In 

 this broad sense leaves are like the lungs of animals, 

 and have a corresponding office. But the processes 

 are not strictly identical, and the leaves have work 

 to do which is not required of the lungs. It is no 

 part of the lungs' duty — at least it is not a primary 

 duty — to get rid of the superfluous water from 

 the system; that is done by the skin and kid- 

 neys. On the other hand, it is one of the chief 

 duties of the leaf to exhale and evaporate super- 

 fluous moisture. The juices are modified by the 

 changes which take place in the leaf, according 

 as certain gases are absorbed or exhaled, and ac- 

 cording as a larger or smaller quantity of water is 

 evaporated from their surface. But this is not all ; 

 the leaf is not only exposed to atmospheric influence ; 

 it is during the day-time subjected to the influence 

 of light, of course conjoined with heat ; and this com- 

 bined effect of light and heat produces a series of 

 changes and transformations in the juices of the 

 plant to which there is no parallel in the animal 

 kingdom, unless perhaps in the lowest groups, which 

 partake, to some extent, of the characteristics of 

 both Idngdoms, but which may be left out cf con- 

 sideration for our pui-pose. A leaf, then, unites 



