CHAP. 11. ROOT. 25.3 



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 whicli 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 which 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 Orchideee the 



