40 OUR WOODLAND TREES. 
surface, where they unfold, and spread out their 
surface to the light, under the influence of which, 
by a process which will be subsequently explained, 
they change their colourless appearance to green, 
and fulfil for the plant in this, its early stage of 
growth, the same offices as are, at a later period, 
discharged by the leaves of the more advanced 
after-growth. 
A stem has now been formed by the pro- 
longation of the plumule: and the unfolding 
of successive leaves, as this stem continues its 
progress upwards, powerfully aids the growth of 
the young plant; for the leaves, as we shall sub- 
sequently see, obtain from the atmosphere a large 
proportion of that which constitutes the bulk of 
the plant’s substance. 
It is, of course, assumed that the beautiful pro- 
cess which has been described, continues its opera- 
tion only under the continued influence of the con- 
ditions which occasioned its commencement—con- 
ditions which are naturally brought to the mind by 
the mention of the word ‘Spring,’ so suggestive as 
that word is of soft rain and gentle warmth. The 
temporary absence of sufficient warmth would 
