Development of Roots of Agricultural Plants. 429 
salts, it might possibly be the case that though one plant appro- 
priates less than another, it may really have imbibed as much, 
as it has means of getting rid of the excess ; and this would apply 
to tree parasites as well as to other plants. 
On this, as on most other questions of vegetable physiology, 
diametrically opposite opinions have been current, some attri- 
buting the deterioration of land for especial crops as much to 
the poisoning of it from excretions as from the consumption of its 
nutriment 
The matter has lately been studied by Cauvet, an abstract of 
whose memoir I have given in the ' Gardener's Chronicle ' for 
Feb. 14, 1863. His observations agree very closely with those 
contained in a lecture on Agricultural Chemistry by Dr. Daubeny, 
which is reproduced in the agricultural portion of the same 
journal for Dec. 14, 1861, every sentence of which may be read 
with profit. 
Most previous experiments had been made with plants more 
or less injured, if not by actual division, for the purpose of 
inserting different portions of the plant in different fluids, at least 
by the destruction of the spongelets, the only active parts, due to 
the removal of the plant from the earth, or the mere act of washing 
away the adherent soil, which could scarcely be effected without 
more or less impairing such delicate organs. To avoid this evil, 
M. Cauvet caused seeds to germinate on a wooden frame pierced 
with holes, so that after they had germinated, they could readily 
be lifted up for inspection, without injury, from any fluid in which 
they might be immersed. Now it appeared, first, that fluids in 
which colouring matters are merely suspended were wholly unfit 
for the purpose, as they would not enter into the tissues. If 
matters were employed which were really soluble in water, — 
whether active, as solutions of minerals — or inactive, as the juice 
of Phytolacca berries, — they were received only partially into the 
system, so long as the spongelets were physiologically sound, 
though they affected their cells and the more superficial parts. 
The former substances at once destroyed the spongelets, and if 
admitted at all were admitted by mere capillarity ; while the 
colouring matter of those of the second class was deposited in a 
glutinous form round the cells, rendering the admission of any 
fluid difficult. In the former case the noxious matter entered 
by means of the fibro-vascular system, a circumstance of immense 
importance as regards those diseased conditions of plants in 
which the first symptoms of decay are the deposition of dark 
matter, conMsting probably of ulmates and humates, in the vas- 
cular tissue, as is so frequently the case in turnip rot. The same 
appeaiance is not uncommon in the potato disease ; but as we 
