THE LIFE -HISTORY OF PLANTS. 



215 



and buds are at this period centres of acti\"ity, so the 

 flow is mainly an iipwar^l orn:. In autiunn, when 

 consulidation of ti-s;ii.- lavl storage of nutritive 

 matters ai-e the chief iJi;.Lr:itions of the plant, there is 

 an increased necessity iLir the presence of matters 

 foi-med in consequence of leaf-action, and the flow is 

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

 is 110 al'S 'lute ditierence between crude and elabo- 

 rated ;ap.-. and no absolutely fixed coiu'se for them to 

 take. The ascending sap consists of water pumped 

 up from the roots to supply the requii'ements of the 

 grc'win:^' rrll- an-l tli-; L-heniical changes going- on in 

 them, and tliL execs- of it is evaporated when the 

 leavr- .:xpanl; but, though mainly watery, it con- 

 tains 5-aniL' 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 explaincii : 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 ccintroversy. The assigned 

 causes are d:ti;u;ion. before explained ; capillary at- 

 traction, or the a:lhesion 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 the bubbles of air 

 mixed with the fluid, which in their expansion push 

 the fluid before them : the swaying to and fro of 

 branches, itc. T^Tien so many explanations are 

 given, it is pretty ^'ertain that we have not yet 

 andved definitely at the true one. 



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

 creasing temperature stimulates 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 witliin the tissues 

 becomes consequently compressed. If now the 

 jdant be wounded, the sudden expansion of the an- 

 i:)ushes 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 

 wholly fanciful, as the fluid in question has few of 

 the characteristics of those whose names it bears. 

 The ascending 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 L-iSG lb. from November to May") con- 

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

 solid matter, eleven-tweKths of which latter con- 

 sisted of sugar. The sugar had probably been 

 washed out fi-om the cells dirring 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 

 IMaple-sugar) would be productive of much more 

 serious results than is usually the case, for the fears 

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

 Vine are, if not wholly without foimdation, at least 

 not proportionate to the observed conseqiiences. 



Tissues through -which the Sap Passes. — 



The so-called ascending- sap has been ascertained to 

 pass up thi-ough the soft yoimg wood, either thi-ough 

 the ca-^ities of the cells, passing from one to the 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. ITnder certain circumstances, no doubt, 

 leaves, or even stems, may themselves absorb water 

 or watery vapour dir-ectly from the atmosphere, as 

 is proved by the efltects of s^nring-ing after a hot dry 

 day ; but for the present purpose water so obtained 

 may be disregarded, and -v>'e 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 Kke the limgs of animals, 

 and have a corresponding ofiice. But the processes 

 are not strictly identical, and the leaves have work 

 to do wliich is not reqrdred 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 ha. 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 siu-face. But this is not all ; 

 the leaf is not only exposed to atmospheric influence ; 

 it is dm-ing the day-time subjected to the influence 

 of light, of course conjoined ^dth heat ; and this com- 

 bined effect of light and heat produces a series of 

 changes and transfomiations 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 Iringdoms, but which may be left out of con- 

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



