90 



FOREIGN NOTICES. 



discharge was perfectly visible to the naked eye, 

 the assistance of the microscope enabled me to 

 see distinctly the downward passage of the sap, 

 and that through all the root cells, at a time 

 when physiologists have generally agreed it has 

 its greatest ascending power." 



No doubt such circumstances are of constant 

 occurrence, although unobserved. What gives 

 the cases now brought forward so striking a cha- 

 racter, is the extraordinary extent to which in 

 them the bleeding occurred. 



When stems bleed the observer is in no way 

 surprised; firstly, because he is accustomed to 

 the phenomena; and secondly, because he knows 

 that roots draw fluid out of soil, and send it tip- 

 wards. But when roots bleed, the ordinary ex- 

 planation of the phenomena is no longer applica- 

 ble; for roots cannot be said to draw fluid from 

 the soil when they are removed from the soil ; nor 

 can it be admitted that sap is sent upwards when 

 we see that it runs downwards. 



In explaining the phenomenon of root bleeding, 

 the first step is to consider why sap ascends. 

 This was in part demonstrated more than a cen- 

 tury since by our countryman Hales. In dis- 

 cussing the qnestion of the circulation or non-cir- 

 culation of the sap, this great experimentalist 

 uses the following words: " We see in many of 

 the foregoing experiments, what quantities of 

 moisture trees do daily imbibe and perspire. Now 

 the celerity of the sap must be very great, if that 

 quantity of moisture must, most of it, ascend to 

 the top of the tree, then descend, and ascend 

 again, before it is carried off" by perspiration. 

 The defect of a circulation in vegetables seems in 

 some measure to be supplied by the much greater 

 quantity of liquor which the vegetable takes in, 

 than the animal, whereby its motion is accelerated ; 

 for by Experiment 1st, we find the sunflower, 

 bulk for bulk, imbibes and perspires 17 times more 

 fresh liquor than a man every 24 hours. Besides, 

 nature's great aim in vegetables being only that 

 the vegetable life be carried on and maintained, 

 there was no occasion to give its sap the rapid 

 motion which was necessary for the blood of ani- 

 mals. In animals, it is the heart which sets the 

 blood in motion, and makes it continually circu- 

 late; but in vegetables, we can discover no other 

 cause of the sap's motion but the strong attrac- 

 tion of the capillary sap vessels, assisted by the 

 brisk undulations and vibrations caused by the 

 sun's warmth, whereby the sap is carried up to 

 the top of the tallest trees, and is there perspired 

 off" through the leaves ; but when the surface of 

 the tree is greatly diminished by the loss of its 

 leaves, then also the perspiration and motion of 

 the sap is proportionably diminished, as is plain 

 from many of the foregoing experiments; so that 

 the ascending velocity of the sap is principally 

 accelerated by the plentiful perspiration of the 

 leaves." 



The sap then ascends in consequence of an at- 



tracting force exercised from above downwards 

 by the foliage of plants. But it is evident that 

 this is only a partial explanation of the phenome- 

 non ; for it does not account for the ascent of sap 

 in winter when leaves are absent. In order to 

 explain that fact we must have recourse to the 

 action of endosmose, a force the elfect of which is 

 to produce propulsion. A tree may be assumed 

 to be a combination of hollow tubes freely com- 

 municating with each other, and enclosed in a 

 skin through which JIuids are capable of being ab- 

 sorbed on the one hand, and expelled on the other. 

 If we conceive a body of this kind, in which the 

 tubes are nearly empty, to have its lower ex- 

 tremity plunged in water, the absorbing power of 

 the skin at that part will begin to introduce the 

 water into the interior, and this continuing to go 

 on for a sufficient time the tubes must necessarily 

 become at last filled with water rising upwards 

 from below. To effect this no attracting force 

 at the upper end of the cylinder was necessary; 

 every particle of water which w-as absorbed by 

 the lower end, having driven before it a corres- 

 ponding volume of the water previously existing 

 in the apparatus. Under the influence of this 

 operation the tubes would in time become full, 

 and if unelastic the introduction of more water 

 would be impossible. But if such tubes and the 

 skin that encloses them were elastic and extensi- 

 ble, then any such further quantity of water might 

 be introduced as the apparatus could receive with- 

 out bursting. If we then suppose that the one 

 end of the apparatus were cut open, the sides of 

 the tubes would collapse, and the water would be 

 forced out till there was no more left than the 

 tubes held in their original unstretched condition. 



A tree is just such an apparatus. Its tubes 

 are nearly empty at the fall of the leaf. During 

 winter the roots absorb water trom the soil and 

 fill the tubes again. By the arrival of spring 

 they are tilled almost to bursting, and then if the 

 stem is cut it bleeds ; or if the roots are cut they 

 bleed. 



Bleeding ceases as the leaves unfold; the vine, 

 the walnut, the birch, are all as incapable of 

 bleeding as other trees when their leaves are 

 formed; because the leaves gradually empty the 

 tubes, put an end to their distension, and prevent 

 its recurrence so long as they remain in an active 

 state. 



The excessive loss of sap mentioned in the 

 cases that have produced these remarks could nc' 

 have taken place if the roots had been wounded 

 in the summer or attumn ; and if the trees sur- 

 vive, bleeding will cease with the appearance of 

 leaves. It is probable, however, that it has been 

 increased by the coldness of the spring. Hales 

 himself was aware that sap falls back at night in 

 consequence of the contraction of the tubes by 

 cold ; Mr. Knight observed the same fact : and 

 it has more recently been proved experimentally 

 by M. BiOT. It may therefore be supposed that 



