ROOTS, SAP. 23 



We have already said incidentally, that in order that a «eed may 

 germinate, it must be in contact with moisture, have communication 

 with the air, and be under the influence of a certain temperature. 

 The same conditions continue to be indispensable after the seed has 

 sprung, and the plant has been organized ; and in addition the access 

 of light is now imperative. 



Roots seek in the soil the moisture which is requisite to vivify 

 the whole vegetable. These organs are terminated by hair-like 

 fibres of extreme delicacy, and having sprngioles at their extremities : 

 it is by these spongioles that absorption is effected. The following 

 experiment is suliicient to prove that this is the case : let such a 

 plant as a turnip be placed with the hair-like extremities of its root 

 plunged in water, and the plant will continue to live, although almost 

 the whole body of the root is in the air ; let things be now so ar- 

 ranged that the hair-like filaments of the root are not in the water, 

 but that the bulb or body of the plant is so : the leaves will soon 

 droop and wither. 



The force which brings into play the suction power of the roots, 

 resides in almost every part of the plant : thus a root deprived of 

 its spongioles, a stem, a branch, and a leaf, exert this suction power 

 when plunged in water. But the absorption effected in this way 

 has a limit, and we soon discover the necessity of making fresh 

 sections of the extremities, which have no power of renovation like 

 the filaments furnished with spongioles, which terminate a root. 



We are still ignorant of the cause which produces the ascent of 

 liquids in vegetables, and which carries them to the remotest leaves, 

 in spite as it were of the laws of hydrostatics. We readily conceive 

 how the spongioles of the roots, surrounded by earth abundantly 

 charged with moisture, should imbibe by the simple effect of poro- 

 sity. We can also understand how, after having been modified by 

 the spongioles, the water and the principles contained in it should 

 be transformed into sap ; but the porosity of the extremities of the 

 roots, and the chemical modification effected by the spongioles upon 

 the fluid imbibed, give no kind of explanation of the rapid ascent of 

 the sap. The force which occasions this rise is very considerable, 

 as was demonstrated by Dr. Stephen Hales in a series of ingenious 

 experiments more than a century ago. 



Hales adapted a tube bent at a right angle and filled with water, 

 to the extremity of the root of a pear-tree, the point of which had 

 been cut off; the extremity of the tube opposite to that which was 

 connected with the root dipped into a bath of mercury. In a few 

 minutes a portion of the water contained in the tube was absorbed, 

 and the mercury rose above the surface of the bath to the extent of 

 eight inches. In the beginning of April, Hales cut off a vine stem 

 at the distance of thirty -three inches from the ground. The stem 

 had no lateral branches, and its cut surfiice, which was nearly cir- 

 cular, had a diameter of ^ths of an inch. To this section, he adapt- 

 ed a reversed syphon : and things being so disposed, he poured in a 

 quantity of mercury, which after a time, and from the effect of the 

 pressure exerted by the sap as it escaped, rose in one of the arma 



