42 BUDS ON LEAVES. 



young plants sprout from the leaf and may be transplanted as independent 

 growths. We will briefly describe what takes place. 



The first change observed in a leaf which has been cut off for the purpose 

 of forming cuttings is the desiccation of the cells lying next the cut surface. 

 Beneath the layer of dried-up cells a cork-tissue is formed, whilst the dead, outer 

 layer is converted into a bark. A parenchymatous tissue is next formed from 

 the part beneath the cut which is still living; indeed, it is the epidermal cells 

 nearest to the dead layer of cells that initiate this formation of tissue. They 

 grow in a radial direction, elongating and dividing by means of the insertion of 

 transverse walls, the result being a uniform thickening coextensive with the 

 surface of the wound. A little later some of the living cells in the middle of 

 the cut, which are still covered over by the dead layer, begin to divide; and as 

 the tissue there grows in size, it tears the overlying dried layer into shreds and 

 pushes it off ia parts. This exuberant tissue has received the name of callus. 

 Whilst the formation of callus is proceeding, suckers are developed at the points 

 of contact of the leaf-cutting with the sand, their numbers being particularly 

 abundant along the projecting ribs of the leaf. In form and function these 

 suckers are entirely similar to the absorbent cells lying close to the growing 

 extremities of roots, and called root-hairs. They are of the greatest importance 

 to the leaf-cuttings in their subsequent processes of development. So long as 

 the leaf adhered to the axis it was supplied with a sufficient quantity of watei 

 from that which was ascending through the stem; the aqueous vapour lost 

 through evaporation was replaced by moisture absorbed by the roots from the 

 damp soil and afterwards conducted through the stem to the leaf in question. 

 But when the leaf has been cut off it is no longer able to derive any material 

 from the earth through the intervention of the stem, and as its ordinary epidermal 

 cells have not the power of taking up from the damp soil, which serves as sub- 

 stratum to the leaf-cutting, as much water as is lost by evaporation, the cutting 

 is exposed to the risk of desiccation in spite of its being in contact with a wet 

 substratum. In order to escape this danger and save itself from destruction 

 the leaf treated as a cutting furnishes itself with absorbent cells. By their 

 instrumentality the water, which is particularly needful for the formation of 

 callus, is put by. Even if the materials necessary for the construction of the 

 cells of the callus may be present in abundance in the cells of the leaf, it is of 

 little avail unless these materials are diluted and conducted to the places where 

 they are used up, and for this a much greater quantity of water is requisite 

 than could be retained by the severed leaf. When the callus has reached a 

 certain size numerous roots make their appearance. They usually take their rise 

 from cells of the parenchyma adjacent to a vascular bundle of the leaf, break 

 through the callus, and grow rapidly in length. Only after the development of 

 these roots, which absorb liquid copiously from the substratum by means of 

 their suction-ceUs, are buds produced on the upper — ^less frequently also on the 

 under — surface of the leaf -cutting. In Begonias it is chiefly cells of the epidermis 



