582 LECTURE XXXriT. 



The pressure to which the tissue-cells are normally subjected, and which prevents 

 their free growth, may easily be relieved artificially on one side by cutting across a soft 

 stem or leaf-rib, or even a root, and then surrounding the cut surface with water or 

 damp earth ; in this case there is formed, in many species of plants at any rate, if 

 not always, the so-called callus, which consists essentially in that all the cells of 

 the epidermis, cortical parenchyma, cambium, &c., which are still vitally active, now 

 grow forth from the cut surface and divide, and so form a cushion of soft tissue which 

 at length covers the surface of the section : this cushion is the callus. These callus- 

 cushions have moreover the highly remarkable property of giving origin to new 

 growing-points of shoots and roots, from which new plant-individuals become 

 developed. The callus-cushions can be obtained with the greatest ease if large pieces 

 of branches, as thick as the arm, of Poplars and many other trees, are cut off and cut 

 smooth above and below, and kept during the winter in a moist warm chamber : a 

 thick swollen callus arises at the boundary between the cortex and wood. The same 

 thing happens if fresh living foliage-trees are cut across above the soil. In such cases 

 dozens or even hundreds of young leaf-shoots are often seen to sprout forth from the 

 callus formed on the cut surface of the rooted portion. 



Hansen^ has investigated in detail the formation of the callus on the cut-off leaves 

 and flower-stalks oi Achiinenes grandis, leaves oi Begonia rex and shoots oiPeperomia. 

 These organs planted with the cut surface in water, or moist sand or soil, develope 

 the callus there. The section by which the ' cutting ' is separated from the parent plant 

 is the external stimulus to the commencement of processes of growth on the surface 

 of the wound. In the first place the layer of cells bounding the cut surface perishes : 

 this dead tissue is frequently separated from the living tissue still further by the 

 formation of several layers of cork, a process also common elsewhere, and which may 

 be produced by wounding masses of tissue which are still living. Beneath this 

 protection there now begins an active growth of all the tissue-elements still provided 

 with protoplasm : i. e. the cells of the epidermis, collenchyma, and cortical tissue. 

 The epidermal cells here perform in addition a peculiar function, growing out into 

 long root-hairs which supply the young callus with water and nutritive salts. The 

 callus grows not merely from the cut surface, but also extends laterally, so that the 

 stem of a leaf-cutting often swells up to double its original diameter, and the callus 

 as a thick cushion includes the cut end of the petiole. This mass of tissue at first 

 consists of nothing but similar parenchyma cells ; but after some time numerous 

 vascular bundles are differentiated in it, and extend in all directions towards the 

 surface of the callus. At various spots on the tissue of the callus superficial cells now 

 become rich in protoplasm, divide actively, and produce a small-celled embryonic tissue, 

 which is conspicuously marked off from the surrounding tissue of the callus. These 

 embryonic masses of tissue or growing-points soon become elevated, produce leaves, 

 and thus emerge as young shoots. At the same time embryo roots develope in the 

 interior of the callus ; these break through the tissues, penetrate the soil, and con- 

 tribute to the nutrition of the new shoots. 



Begonia leaves cut off and laid flat on damp sand exhibit still other curious 



* Hansen's researches on the formation of callus are found in his paper, ' Vergl. Unters, über 

 Adventivöildtmgen,' in Abb. der Senkenbergischen naturf. Ges., B. XII, 1881. 



