" * s 3 ale’ Fs 
~ ¢ \ ‘ 
A . 4 an + 
192 Structural and Physiological Botany. 
neck, either remaining simple or splitting up into several 
contiguous cells. The central cell developes into a large 
oosphere, which is surrounded by the neighbouring tissue of 
the endosperm as a peculiar enveloping layer of cells. The 
pollen-tube now advances to the corpuscules, becomes. 
closely apphed to several of them, or penetrates into the 
neck of a single one, often even into the central cell, and 
transfers its contents to the oosphere. ‘This is the moment 
of fertilisation ; and the nucleus of the oosphere now be- 
comes absorbed. ‘The oosphere now breaks up in its lower 
part, in Cupressineze, into several cells lying one over 
another ; or, in Abietineze, a layer of cells is developed there 
by repeated cell-divisions. These cells form the so-called 
pro-embryo, which in the course of its development breaks 
through the embryo-sac, the embryo being formed at its 
lower end. When the rudiment of the embryo is carried 
into the loosened endosperm by the pro-embryonic cells 
which are often greatly elongated, the corpuscules gradually 
coalesce before their contents are completely absorbed and 
consumed. 
The germination of many seeds takes place as soon as 
they escape from the fruit and attain favourable conditions 
of moisture and warmth; in others, on the contrary, a 
period of rest seems to be necessary in order for them to 
ripen, z.¢. to become capable of germination, by slow 
changes, probably of a chemical nature. The radicle of a 
seed of the willow pierces the testa as soon as twelve hours 
_ after sowing ; and if it is allowed to remain dry for only 
twelve days it loses its capacity for germination. The seeds 
of the elm, poplar, coffee, and many Lauracez also germi- 
nate only when they have been kept fresh and have not 
dried up ; while that of the mistletoe often germinates even 
while still inside the fruit. Fresh seeds in general germinate 
more quickly and easily than old ones. Seeds which con- 
tain starch may, on the contrary, retain their power of ger- 
mination for many years, if kept dry. The temperature 
