198 PRINCIPLES OF PLANT CULTURE 



of roots is stimulated. Indeed bottom heat often excites 

 growth in cuttings that will not grow without it. 



A comparatively low air temperature is important in 

 growing many plants from cuttings of the stem (377), 

 because it is essential that the stem growth be held in 

 check until roots are formed. A soil temperature of 

 about 65° F., with an air temperature about fifteen degrees 

 lower, is suited to the layer member of plants usually 

 propagated under glass from cuttings. It is important 

 that these temperatures be maintained nearly constant 

 until roots have developed. 



Since we have better facilities for raising than for 

 lowering the natural temperature of the atmosphere, 

 propagation from cuttings is easiest at a time of the 

 year when the temperature of the atmosphere during the 

 day does not much exceed 50°. By observing special 

 precautions, however, it is possible to propagate many 

 plants from cuttings during the warm season. 



Abundant moisture is important in growing plants 

 from cuttings, because moisture favors root develop- 

 ment (88), and water is essential to cell growth (62). 

 The amount of water required varies considerably with 

 different plants and conditions. 



With cuttings containing leaf tissue (377, 382), 

 transpiration (74) must be reduced to the minimum 

 until roots are formed, because water cannot be taken 

 up freely without root-hairs (100). For such cuttings, 

 therefore, the air as well as the soil must be kept abun- 

 dantly moist (369), and the direct rays of the sun must 

 be intercepted by shading (235). 



363. Methods for controlling temperature. — The 

 alternations of temperature in the open air are un- 

 favorable to the development of cuttings, though many 



