38 BOTANY 
the blade a sharp tap. The seed splits and sets free 
the embryo. It is difficult, even with a lens, to dis- 
tinguish cotyledon, radicle, and plumule. Soak the 
split endosperm in weak iodine solution. The fact 
that there is no blue coloration shows that starch is 
absent. Now place on the split surface a drop of 
strong sulphuric acid and the blue colour which 
indicates the presence of cellulose immediately appears. 
Speaking broadly, we have now arrived at the 
conclusion that the seed is really a young plant which, 
together with its store of nourishment, has been 
formed from the ovule of the flower while still attached 
to and forming part of the parent plant. At a certain 
stage, the development of the embryo plant ceases, and 
we say that the seed is ripe. All growth then stops, 
and the seed, becoming dry, goes into a resting con- 
dition. This temporary arrest of development is an 
obvious advantage. The seed ean, to some extent, 
choose its own time for renewal of growth. If, in the 
autumn when most of the seeds fall from the parent 
plants, the embryo were still growing, it would mean 
that the young plant would have to establish itself 
under adverse conditions. Autumn, in many places, 
is the driest season of the year, and the tiny seedling 
would thus have to contend with scarcity of water, 
and later on be overwhelmed by the frosts and snows 
of winter. Instead, the embryo remains dormant till 
the warm spring rains supply the conditions most 
favourable to its growth. 
The Mangrove, which grows on the mud-flats of the 
tidal inlets of the Auckland Peninsula, is modified to 
suit its peculiar environment. If the seed were dropped 
into the mud in an absolutely dormant state it would 
probably be carried out to sea before it had a chance 
to germinate. To avoid this, most mangroves show 
no arrest in the development of the embryo, growth 
being continuous from ovule to fully established tree. 
