CHAP. II. 
ROOT. 
253 
of the shadow, and extend wherever moisture is to be found. 
This property prevents a plant from exhausting the earth in 
which it grows ; for, as the roots are always spreading further 
and further from the main stem, they are continually entering 
new soil, the nutritious properties of which are unexhausted. 
It is generally believed that roots increase only by their 
extremities, and that, once formed, they never undergo any 
subsequent elongation. This was first noticed by Du Hamel, 
who passed fine silver threads through young roots at differ- 
ent distances, marking on a glass vessel corresponding points 
with some varnish : all the threads, except those that were 
within two or three lines of the extremity, always continued 
to answer to the dots of varnish on the glass vessel, although 
the root itself increased considerably in length. Variations in 
this experiment, which has also been repeated in another 
way by Knight, produced the same result, and the whole 
phenomenon appears to be one of those beautiful evidences of 
design which are so common in the Vegetable Kingdom. If 
plants growing in a medium of unequal resistance lengthened 
by an extension of their whole surface, the nature of the me- 
dium in which they grow would be in most cases such as the 
mere force of their elongation would be unable to overcome, 
and the consequence would be that they would have a 
twisted, knotted, unequal form, which would be eminently 
unfavourable to the rapid transmission of fluid, which is their 
peculiar office. Lengthening, however, only at the extremi- 
ties, and this by the continual formation of new matter at their 
advancing point, they insinuate themselves with the greatest 
facility between the crevices of the soil ; once insinuated, 
the force of horizontal expansion speedily enlarges the cavity ; 
and if they encounter any obstacle whicli is absolutely insur- 
mountable, they simply stop, cease growing in that particular 
direction, and follow the surface of the opposing matter, till 
they again find themselves in a soft medium. 
It is curious, however, to remark that, although this proper- 
ty of lengthening only by the ends of their roots seems constant 
in most plants, yet that it is not impossible that it may be 
confined to roots growing in a resisting medium. From the 
following experiments it will be seen that in Orchideae the 
