The Life of the Plant. 2 NOB 
rises during the germination of seeds, in consequence of the 
absorption of water and formation of carbon dioxide. 
In order for germination to take place, it is essential 
that the temperature should fluctuate only between certain 
limits, which, as might be expected, vary greatly in the case 
of different plants. In the case of wheat, for example, the 
lower and upper limits are 5° and 43° C.; for barley 5° and 
36°-37'5° C.; for maize 15° and 46:2° C. The seedling 
derives its ms nutriment from the 
seed, and especially from the cotyledons, 
or, when there is endosperm, from it also 
through the medium of the cotyledons. 
The radicle first emerges from the seed, 
and after it the plumule. The 
radicle is, in Dicotyledonous 
plants, a direct prolongation 
of the axis of the embryo, and, 
therefore, a tap-root; while 
in Monocotyledons, on the 
other hand, the radicular end 
of the embryo never becomes 
a primary root; a number of 
adventitious roots are, on the 
contrary, developed (see Fig. 
£05,-p- 74)),.and it is only 
rarely (Fig. 368) that the ra- 
dicular end of the embryo 
elongates into a protuberance 
wie satn perishes: «Im te- pc, oot P'S. 39 7 Sore 
mination of the ing seed of orange, 
oat ; tyl ledons 
spect to the cotyledons also, don ; serie of Tee ten ae 
there are two modes of germi- the embryo; @ — soil,and only slightly 
: ? ; a re tadicle 2/7 pliz emerging from the 
nation. ‘They either remain mule. (x 6.) testa T. 
beneath the sou still enclosed in the testa, (Fig. 369), or 
they rise above ground and throw off the testa (icy 270R 
The single cotyledon of Monocotyledons almost always 
remains beneath the surface, as also do those of the oak 
O 
