NUTRITION OF VEGETABLES; 



49 



teluperatui-e is singularly favourable to the pro- 

 gi'ess of the sap. In winter, the tree is full of 

 sap, but it is thick and stagnant. In spring, the 

 return of heat causes the ascent of the juices in 

 the vessels of the stem, which seemed to be 

 obstructed by them. Light and the electric 

 fluid have also a decided influence upon the 

 phenomena of the progress of the sap. It is well 

 known that when the atmosphere is long charged 

 with electricity, vegetables acqnire a great de- 

 velopment, which necessarily implies that the 

 sap moves with more rapidity and power. Cer- 

 tain internal causes, inlierent in the vegetable 

 itself, appear also to act upon the ascent of the 

 sap. Of this kind are the greater or less quantity 

 of cortical pores which the vegetable presents, 

 and the greater extent of its surface. These two 

 circumstances are evidently favourable to the 

 rapidity and force of the progress of the sap. 



We now proceed to give a summary of the 

 theory of the ascent of the sap, as proposed by 

 Dutrochet, and illustrated by numerous experi- 

 ments by this ingenious botanist. In the first 

 place, Dutrochet has confirmed prior experi- 

 ments, that the imbibition of moisture from the 

 soil is accomplished solely by the extreme 

 rootlets or spongioles of plants. At the com- 

 mencement of spring, he performed in the vine 

 successive amputations from the branches do^vn- 

 wards to the stem roots, and extreme radicles. 

 At each of these amputations the sap ceased to 

 flow from the upper surface of the separated 

 portion, and proceeded to issue from the part of 

 the plant next the spongioles, till at length he 

 arrived at those spongioles themselves, from 

 which the sap issued in a similar manner. These 

 spongioles are internally composed of a cellular 

 tissue, in which are found minute corpuscles, 

 and covered by an exceedingly thin bark, which 

 in autumn becomes thicker and impervious, and 

 consequently loses its absorbing function. As 

 spring approaches, the spongioles are renewed in 

 the form of buds, from the extremities of those 

 of the previous season. The ascent of the sap 

 at first view, appears to be nothing more than a 

 mechanical action of imbibition carried on by 

 the leaves, as it increases and diminishes in 

 amount with the evaporation from them, and 

 diminishes gradually if they are removed one 

 after the other. But in a more careful inquiry, 

 it will be seen that this mechanical action alone 

 will not account for aU the appearances. For 

 in the first place, in the sunshine, or in a diffuse 

 light, the absorption by the roots does not equal 

 the transpiration by the leaves ; and if the plant, 

 after losing weight in the light, be removed into 

 darkness, the transpiration then will not equal 

 the absorption. Secondly, although the flaccid 

 state of a plant which has been deprived of 

 moisture, may be rapidly succeeded by the 

 healthy state of turgidity, on an adequate supply 



of water being restored ; nevertheless, this re- 

 %'ival will only take place, provided the desicca- 

 tion has not been carried too far; and the limit 

 of desiccation at wliich revival wiU no longer 

 occur, on the one hand, accords with that at 

 which the organized structure of the plant be- 

 gins to be decomposed, and on the other hand, 

 is much within that at which the plant is to be 

 considered in the light of a sponge, reduced to a 

 state of dryness, and thereby deprived of its 

 power of capillary attraction. Neither can the 

 ascent of the sap, as some have supposed, be owing 

 to air contained in the vessels, which expanding, 

 forces the fluid upwards, for the sap vessels, when 

 cut, exliibit no traces of contained air ; nor can 

 it be propelled by a contraction of the vessels, 

 for those in the stems of ligneous plants are of 

 an unyielding and solid nature. In short, Du- 

 trochet concludes that the ascent of the sap is 

 due to a peculiar impulse inherent in the tis- 

 sue of the plant, and which is dependent on the 

 integrity of its organic structure. — We proceed 

 then to detail Dutrochet's experiments explan- 

 atory of his theory. 



When a piece of the blind gut or coeoum of a 

 chicken was half filled with milk, and then im- 

 mersed in rain water, he found that it became 

 gradually fuller and fuller, and at length very 

 turgid, having in thirty-six hours increased in 

 weight, from 196 to 313 grains; when a denser 

 fluid was substituted for the milk, such as albu- 

 men or solution of gum, the weight and tur- 

 gescence, were still more increased, and tliis 

 increase was more rapidly completed. In eight 

 and a half hours, a ccecum, partially filled, and 

 weighing fifty-eight grains, became extremely 

 turgid, and weighed 130 grains. This trans- 

 mission of the water by inward impulse he has 

 teiined endosmose, and corresponds to the imbi- 

 bition of the moisture of the soil by the roots. 

 It occurs always when the internal fluid is more 

 dense than the external, as also under certain 

 other circumstances to be mentioned afterwards. 



On the other hand, when the eoecum was filled 

 with rain water, and immersed in any of the 

 above mentioned fluids, such as milk or albu- 

 men, the water passes outwards, through the 

 membrane; and a weaker solution of gum arable, 

 inclosed in the same way, passed outwards to a 

 stronger solution in which the gut was immersed. 

 This process he termed exosmose, or outward 

 impulse ; and it corresponds to the imbibition of 

 moisture from the external atmosphere into the 

 interior tubes of plants. But these two pro- 

 cesses of endosmose and exosmose, are always 

 reciprocal to a certain degree : thus, while a 

 portion of one fluid passes inwards, a certain 

 portion of the interior fluid also passes outwards. 

 When a little syrup is inclosed in a membran- 

 ous bag, and immersed in pure water, the water 

 enters the bag; but at the same time, a part of 

 a 



