FORMATION OF CALLDS. 



583 



phenomena. In the first place, the section which separates the leaf-stalk not only 

 induces the formation of callus, shoots and roots at the wound, but also at a place 

 some distance from it, namely where the petiole passes over into the lamina, and 

 where at the same time the primary ribs of the leaf arise. If the nerves in a leaf 

 so treated are cut through here and there, callus-cushions, shoots and roots are like- 

 wise formed at those places. Moreover isolated cells in the epidermis of the leaf- 

 nerves, situated at a distance from the sections, are stimulated in a remarkable 

 way to renewed life. They grow vigorously, division-walls are formed in them in 

 various directions, and finally the small-celled growing-point of a new shoot is 

 developed in them. 



These and many other observations show that a cut made in an organ of a plant, 

 when the vegetative conditions are otherwise favourable, is to be looked upon as a 

 stimulus to growth — as an impulse to extensive and complex activities. In the first 

 case of course the tissue adjoining the section is simply freed from certain hindrances 

 to its growth; and the beginning of the formation of callus depends on this. When 



Fig. 355. — A seedling of ^icia Faia, the root and plumule of which were straight, 

 was so placed that the root-apex lay almost horizontal on the surface of the mercury 

 (black in the figure), and fixed in that portion to the cork A by means of a pin : n « 

 layer of water on the mercury. The figure shows the seedling about 24 hours later. 

 The growing part of the root has curved sharply downwards, so that the apex 

 enters the meicury perpendicularly : the resistance which it meets with is expressed 

 by the form of the root behind the downward curved portion. The stem has 

 become sharply erected at its basal portion ; the nodding position of the bud is a 

 phenomenon of nutation, independent of gravity. 



this has proceeded to a certain extent, however, growing-points arise in the tissue of 

 the callus, which of course are to be regarded, not as for and by themselves immediate 

 effects of the section, but as consequences indirectly due to it. 



While the preceding considerations and facts show that the tissue-cells undergo 

 passive strains and pressures during their growth, on the one hand, and on the other 

 hand are compelled to act in this way mechanically on their environment, it is 

 implied at the same time that by means of the processes of growth work is done (in 

 the mechanical and physical sense of the word) in the interior of the tissues. The 

 store of energy of the growing cells is not entirely exhausted by this work, however, 

 as follows at once from the fact that growing plant-organs can also exert powerful 

 pressures externally, on bodies which come in contact with them. This takes place 

 very generally when the apices of growing roots are necessitated to drive their way 

 into the resisting soil ; they not only have to glide between the small particles of soil, 

 but to- push them asunder. The elongating region immediately behind the growing- 

 point of any root pushes forwards the apex, clothed with its root-cap, and thus drives 

 it between the closely packed particles of soil much as a nail is hammered into a 



