GARDENING AS A SCIENCE. 203 
Mr. Knight was not then aware, nor could he be, of the stupendous discoveries, 
made by Dr. Faraday, of the universal presence of electric action, whenever the 
elements of water are disturbed ; otherwise, he would have been still further con- 
firmed in the luminous view which he had already adopted. 
To those discoveries we have before alluded, and therefore shall only remark, 
that whenever a manure appropriate to any plant whatever is within the 
range of its roots, that manure is gradually, but more or less rapidly decomposed, 
and its constituent elements converted in water and hydrocarbons, during which 
conversion a volume of electricity is disturbed equivalent to the quantity speci- 
fically essential to its combining power, and amply sufficient to propel the raw sap, 
so prepared, through the membranous tissue of the roots ; and thence, according to 
Knight, into the cellular vessels of the trunk or stem. 
The spiral tubes, or system of coiled vessels, were once deemed to be the con- 
ductors of the sap ; though by the earlier physiologists, they were thought to be 
merely air-vessels. Speaking of their position and office, Mr. Knight observes : — 
" They are placed between the passages through which the sap ascends, which 
passages are in fact surrounded by them ; and they are certainly so placed as to 
enable them to act by external pressure. They appear so restless, when suddenly 
taken out, that I have offered a conjecture that they were not formed to be idle ; 
but, not liking to indulge much in speculation in my communication to the Royal 
Society, I only said that they appeared to me to be appendages to the sap- vessels, 
and not to carry any liquids themselves. I took a good deal of trouble to ascertain 
whether the spiral tubes, and those tubes which contain them, were at any time 
filled with liquid ; and I, under all circumstances, uniformly found the tubes empty, 
and the spiral thread dry in all mature — that is full-grown — leaves. The experi- 
ments from which I drew my inferences were, I think, as well conducted as they 
could be. I often adjusted my microscope to the stalk of a leaf, growing in its 
natural position, and in the full execution of its office ; and then, after having cut 
through the epidermis only, I have, by means of a small windlas, with soft woollen 
yarn, severed all connection between the plant and the leaf, except that of the 
spiral tubes ; and, under such circumstances, those tubes were always empty, and 
their threads dry ; and having suffered the leaves to remain exposed to the sun by 
the side of others wholly detached from the plants, all died and dried within the 
same period of time. The spiral threads therefore, I think, do not convey the 
ascending sap." 
If we consider these minute experiments, and view the structure of these coiled 
tubes, their great strength, and elastic power— which any careful person can dis- 
cover by examining the tender green twig of an elder — it will appear pretty 
evident that they must act as springs, and thus aid, mainly, in producing and 
sustaining the graceful rotatory movement of which spray and herbage are suscep- 
tible. Some mechanism of the sort is absolutely required, and none appears so 
entirely applicable as the system of spiral vessels. These vessels may also convey 
