400 



PLANT PHYSIOLOGY 



by cutting and the subsequent formation of roots lead to an 

 intense respiration; therefore, a liberal access of air is required. 

 High humidity is another necessary factor for successful rooting. 

 This prevents the cuttings from drying before they have had 

 time to develop their own root system. This is essential for 

 cuttings that bear leaves. Hence, while they are rooting, cut- 

 tings are often kept in greenhouses or in propagating beds under 

 glass. Some cuttings, like those of willow, root very readily 



within a few days; while others, such as 

 conifers or camellias, require several 

 weeks or even months. 



Cuttings also may be obtained from 

 the roots of certain plants. These are 

 placed in a somewhat inclined position 

 and covered completely with light sandy 

 soil. Adventitious buds will then be 

 formed at their upper ends, which will 

 produce shoots, while the roots will be 

 formed from the lower end. 



A leaf from certain plants may also be 

 used as a cutting. It is planted with 

 its peduncle in moist soil. With many 

 leaves, this will result in the formation of 

 roots on the surface of the cut, just as in 

 Fig. 122. — Rooted leaf of the case of stem cuttings. On account 

 Aucuha japonica (after ^f ^j^g absence of buds, many leaves 



Mohsch). ' 1 • 



produce no aerial organs and are incapa- 

 ble of developing into a new plant (Fig. 122). They may only 

 increase in size somewhat but — what is of particular interest — 

 will live much longer, even for several years; while, on the tree, 

 they might have lived at the most only one or two years. There 

 are a few plants whose leaves are able to form adventitious buds 

 when they are separated from the mother plants. The best- 

 known instance is the leaves of some of the Begonias. If one 

 of their leaves is placed on moist sand and the larger veins are 

 cut through, buds will appear at the jjoints of incision and will 

 later develop into new plants. This is a common gardener's 

 method of propagating Begonias by means of leaves. Some 

 varieties of Cardamine, Bryophyllum (Fig. 123), tomato, the 



