Development of Roots of Agricultural Plants. 431 
against anv excretion from the roots. Foreign matters, whether 
actively destructive to the spongelets or not, if once admitted in 
quantity, soon produced death ; and if in consequence of new 
roots being thrown out the plant survived, the peccant matter 
was in no case thrown off by them. 
Fibrous roots have sometimes enormous power of penetration 
in search of nutriment, — a property which becomes a serious evil 
if the subsoil does not contain the proper salts which are needful 
for the especial exigencies of the plant, or in proper proportions, 
or if they meet with matter absolutely noxious. In the orchard 
many a fruit-tree thrives so long as the roots are superficial ; but 
when they penetrate deeper, nutriment either insufficient or of 
an improper character is received, vitality is impaired, and 
various forms of canker or chlorosis are the consequence. To 
some plants lime is a positive poison, and for this reason rhodo- 
dendrons will not succeed in a limestone district.* 
Sometimes the evil is of a different character. In such crops 
as sugar-beet, which is often grown for several years in succession 
on the same ground, where the roots penetrate deeply, a portion 
is always left behind when they are lifted, and so soon as the 
tap-root of the next jear comes in contact with the old decaying 
portion, it is contaminated, the leaves in consequence flag, and 
the whole plant ultimately becomes unhealthy.! 
Another evil attendant on the elongation of roots arises from 
their penetration into drainage tiles, sometimes completely 
blocking them up, and rendering them useless. Willow and elm 
roots often act in this way ; but herbaceous plants are scarcely 
less mischievous. Even grasses will sometimes clog the drain 
tiles up, and I have seen an instance where the fibres of mangold 
have attained in such a situation a length of some feet. A case 
lately occurred in which the common Equisetum of ploughed 
land, or Besom-weed, % was the cause of mischief. 
Another evil arising from deep penetration, where there is 
stagnant water, is the depression of temperature, which is highly 
injurious. Every cultivator is aware of the benefit of bottom- 
heat within certain limits, and where the ground is deep and 
uniform, as in the best cotton and tobacco grounds, the rootlets 
may find exactly that degree of temperature which is most con- 
genial to them. In cold countries, where the surface is frozen 
* This is singularly evidenced -witliin the space of three or four miles in 
Denbighshire. At Coed Coch rhododendrons are a weed, the district being 
Silurian. At Gwrwch Castle no pains have succeeded in establishing them, 
because the district is one of carboniferous limestone. 
t ' Des Maladies de la Pomme de Terre de la Betterave,' &c. Par A. Payen. 
' Bibliothcque des Chemins de Per.' 
X In some districts this name is applied to species of Anthemis and Matricaria, 
asd aUied genera of chamomiles. 
VOL. XXIV. 2 F 
