202 THE PHYSIOLOGY OF PLANTS 



Begonia Bex and its varieties. The strongest ribs are cut through 

 and the leaves are laid with their under surface on damp sand. 

 If care be taken to prevent the leaf from drying up by keeping 

 the atmosphere saturated, and if the temperature is suflBciently 

 high, it will be noticed that the severed rijas are very soon 

 covered in with callus. The wounds become surrounded by 

 an excrescence of tissue which at first produces only roots. 

 Very soon, however, adventitious buds make their appearance 

 on this tissue. These buds, however, do not produce their 

 own roots, but content themselves with those formed on the 

 callus. 



It is especially interesting that these young shoots arise from 

 one, or only very few, epidermal and subepidermal cells in the 

 neighbourhood of the severed vein ; thus we see that even 

 tissues with very little protoplasm are able to enter again into 

 cell-division if they are well supplied with reserve materials. 

 The new roots take their origin much deeper, generally in 

 immediate proximity to the cambium layer of the vascular 

 bundle, which runs along the vein. But even before the roots 

 are developed the leaf seeks to develop absorptive organs, and 

 we find the epidermal cells near the wound growing out into 

 hair-like processes, which somewhat resemble root-hairs. 



As far as our observations go, we may assume that the for- 

 mation of young plants from leaf-cuttings takes place in other 

 plants in much the same way as in the case of Begonia. 

 Some differences occur, as in some cases {Peperomia) no callus 

 is formed over the wound, but the latter is simply protected 

 by a layer of cork cells. In other cases the young buds are 

 not formed directly from the tissues of the leaf, but arise 

 from the callus tissue {Achimenes). Such differences are, how- 

 ever, unimportant, and may occur in the same species under 

 different conditions of nutrition. 



Among the Monocotyledons it is especially the bulbous 

 plants, the leaves of which are prone to form buds, and can 

 therefore be used as cuttings. It is well known that many 

 liliaceous plants may be propagated by bulb scales ; but it is not 

 generally known that, if properly treated, the green leaves will 

 produce new bulbs. The young buds which arise on the cut 

 end of a bulb scale or of a green leaf originate from the 



