CH. V, 9] ECONOMICS OF ROOTS 259 



a low temperature permits only a slow water absorption, 

 which explains the damage often done to trees and shrubs in 

 early spring by warm weather and high winds while the 

 ground is still cold, if not frozen ; for the resultant wither- 

 ing, or windburn, and browning is caused by fatal drying 

 through excessive transpiration unsupported by sufficient 

 water supply (compare page 48). Much winter-killing of 

 plants is due to the same cause, i.e. a similar exceptional 

 loss of water through lenticels while the ground is still frozen. 

 Thus it becomes a part of good gardening so to place and 

 treat susceptible plants, by selection of sheltered situations 

 or suitable coverings and the like, that they cannot be ex- 

 posed to high transpiration conditions while the soil is chilled. 

 The very different powers of plants to strike roots from 

 cuttings or slips has important consequences in gardening. 

 Some, e.g. the common house "Geraniums," strike root 

 very easily ; others, e.g. most ornamental shrubs, do so with 

 some difficulty, and only when aided by special treatments ; 

 while still others, e.g. our common fruit trees, will not do 

 so at all. The causes of the differences are not fully known, 

 but in general plants of succulent texture, with soft fibro- 

 vascular system and plenty of stored food, strike root most 

 easily. The matter is practically important in two ways, 

 -first, plants which strike root from cuttings can thus 

 be propagated rapidly and cheaply, and second, special 

 varieties or sports can thus be preserved and multiplied 

 indefinitely. The usual treatment of cuttings conforms 

 perfectly to root physiology. The skilled gardener cuts 

 usually just below a node, because the roots start most 

 readily from nodes ; he removes much of the foliage, be- 

 cause too much transpiration would dry out the tissues 

 before the new roots could replace the supply ; he puts the 

 cuttings first in sand, because ample air is the first requisite 

 for the vigorous growth of new roots, but when rooted, he 

 transplants them to loam for a better water and mineral 

 supply. 



