REMARKABLE PROPERTIES OF ROOTS. 773 



roots at the node where the knee-shaped bending takes place, in order to conduct 

 the absorbed nourishment directly to the foliage-leaves on the upper part of the 

 shoot. The actual existence of the part of the plant in question depends upon the 

 formation of such roots in the other cases enumerated above. The cut branches 

 of willows, the cut-up foliage of begonias, the ivy-leaves torn from their stem, 

 &c., would all die if they did not provide themselves with roots. But although 

 it is easy enough to perceive the benefit ensuing from this kind of root-formation, 

 it is very difficult to explain how the mechanical impulse brings about this new 

 production. It has been shown in all these instances cited that contact with 

 a foreign body is an important factor, but it is very puzzling to understand how 

 the deeper cell-layers are stimulated to develop roots by the contact of the 

 epidermis with damp earth, water, and the like, and we must content ourselves 

 with saj'iug that the contact acts as a stimulus, which, when transmitted to the 

 deeper layers of cells, stirs them up to construct roots as a delivei-ance from death. 

 The explanation is still more difficult in cases whei-e the cut parts of the plant 

 develop roots to preserve their life, even without contact with a foreign body. 

 Such a case has been considered earlier (on p. 89), when it was shown that on cut 

 shoots of various species of stonecrop (e.g. Sedum reßexuvi, Boloniense, elegans), 

 which are hung in the air by a thread, roots will develop frora the internodes 

 between the foliage-leaves in places where no roots would normally have arisen. 

 They grow down and elongate until their tips come in contact with some solid 

 body. Here no stimulus could have acted on the epidermis; the pendent shoots 

 stand in no relation to the surrounding air other than obtained whilst they were 

 still united to the rooted plant, i.e. before they were cut ofi'. The stimulus to root- 

 formation must, therefoi'e, be referred to the separation of the shoot from the plant. 

 We must not, however, imagine the action to be merely mechanical, but must be 

 content with stating that the living shoot hanging in the air can only save itself 

 from death by developing these roots. 



To the most remarkable vital phenomena of plants belong also the various 

 bendings, curvatures, and other movements performed by growing roots. Ap- 

 parently every root ti'ies to reach a definite goal, towards which it directs 

 its way, endeavouring to obtain the advantages oflered by it with as little expendi- 

 ture as possible. The goal which growing roots stinve after is the same for all, 

 viz. the place in the nourishing substratum best adapted to them. The pi-imary 

 roots of plants settled on the bark of trees as epiphytes or parasites direct their 

 tips towards the axis of the branch of the tree in question, laud plants on the 

 other hand, towards the centre of the earth, and the primary roots proceeding 

 from seeds lying at the bottom of still water sometimes direct themselves upwards 

 and grow at the commencement of their development towai'ds the surface of 

 the water. The road to be traversed by the succeeding roots, from whatever 

 part of the plant they may have sprung, is apparently not so clearly determined; 

 but on a closer examination it is found that they too strive to attain to places 

 where fluid nourishment abounds, and where they can obtain a firm hold. The 



