Propagation by Cuttings. 187 
majority of plants usually propagated under glass from cut- 
ings. Itis important that these temperatures be maintained 
nearly constant until roots have developed. 
Since we have better facilities for raising, than for lower- 
ing the natural temperature of the atmosphere, propagation 
from cuttings is easiest at a time of the year when the tem- 
perature 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. 
c— Abundant moisture is important in growing plants 
from cuttings, because moisture favors root development 
(89), and water is essential to cell growth (63). The amount 
of water required varies considerably with different plants 
and conditions. 
With cuttings containing leaf tissue (377, 382), transpira- 
tion (75) must be reduced to the minimum until roots are 
formed, because water cannot be taken up freely without 
root-hairs (101). For such cuttings, therefore, the air, as 
well as the soil, must be kept abundantly moist (369), and 
the direct rays of the sun must be intercepted by shading 
(236). 
363. Methods for Controlling Temperature. The 
alternations of temperature in the open air are unfavorable 
to the development of cuttings. Some structure, therefore, 
that may confine warmth radiated from the earth, or artifi- 
cially generated, or that may when necessary shut out a 
part of the solar heat, is always of great assistance in pro- 
pagating plants from cuttings, and in many species, is essen- 
tial to success. Since light is necessary to assimilation (59), 
such a structure must be roofed with glass, or some other 
more or less transparent material. 
