450 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



being thus supplied the mother-plant casts them early. In contact 

 with water as a stimulus they form roots at the expense of food- 

 material in the scale-leaves, and the bud elongates as an incipient 

 shoot. The two factors — food-store and water-stimulus — are present 

 here. 



Ranunculus Ficaria offers an example of the same provision, only 

 an early 'formed root serves as the food-store. 



That such separable propagative bulblets or tuberous bodies of 

 kinds are not nearly so common amongst dicotylous plants as amongst 

 monocotylous ones is a fact of phyletic interest, and it is one, too, of 

 practical import to gardening; without such bulblets, oorms, and the 

 like, our gardens would be deprived of the enrichment they derive ! 

 through the free and rapid propagation of monocotylous plants. 



But Nature proceeds in another way at times. Supposing an 

 injury has been done, say, to a branch by which it is broken off 

 near the base. Nature tries to protect the wound and repair the 

 injury by the familiar process of forming a callus. If the stump of 

 the branch be too long this is ineffectual and rotting passes down the 

 broken branch to the centre of the tree (fig. 152). But if the stump be 

 short (fig. 153), or if we assist Nature by clean cutting the base of the 

 broken branch as sound pruning requires, the callus covers the wound 

 entirely, all trace of which disappears, and from the margin of the 

 healing cushion there may be formed many small shoots — miniatures 

 of the branch that was lost. From the callus formed all round the 

 margin of the clean-cut stump of a tree felled for coppice many stool- j; 

 shoots may arise (hg. 154), and the tree may in time through one or 

 more of these replace the head that has been removed. i 

 This callus is of supreme importance in relation to vegetative. I 

 propagation. It consists of a quantity oi indifferent meristem-cells — j 

 cells, that is to say, which are capable of dividing and multiplying, but 

 whose fate has not been definitely determined. Circumstances and ] 

 relative position will determine that. Callus may arise from any mass 

 of living cells under the stimulus of wounding. i-, 



In an ordinary dicotylous stem or root it may take origin in the 

 pith, in the medullary rays, in the cortex, or in the active wood- 

 cambium, and it forms lobulated projecting masses at the point where 

 it occurs (fig. 155). Perhaps in most cases the wood-cambium is the 

 most important seat of its formation (fig. 156). Callus more rarely 

 forms in monocotylous plants- — they are content to heal wounds by a 

 cork covering only ; when it does appear it arises from the cortex of the 

 stem. Leaf -callus comes from around the veins. 



In all situations callus has the same potentialities. It is a wound- . 

 protecting tissue to begin wifli, but its cells may be absorptive and 

 may also take on the work of restoration of the plant-body by pro- 

 ducing organs that have been removed by wdunding. It is a mark. of 

 the colonial organization of the plant. 



In the light of knowledge of the facts to which I have referred, 

 the gardener has to solve the problem of propagation by cuttings. 



