50 NEW ZEALAND NATURE-STUDY BOOK 
outer portion causes this to shrink up and die. 
The bark 
then becomes thick and tough, and, as the tree expands, 
we lag va p\ 
" ae eS RR \ 
o prt ¢ AAW 
ny 4! er \ 
one 
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Fig. 38.—Oak-stem cut across to 
show the annual rings. The new 
wood (sap-wood) is just below the 
bark. The rays running from pith 
to bark form the silver grain, 
splits from time to time, and 
gradually acquires the irreg- 
ular and wrinkled surface 
characteristic of older trees. 
The bark acts as a water- 
proof covering confining 
transpiration to the leaves 
and green’ parts, thereby 
securing the passage of the 
salt-solutions to these organs, 
and preventing loss in the 
food on its passage from the 
leaves to the growing parts. 
UNDERGROUND STEMS. 
In the case of many plants the whole of the stem does 
not rise above the ground, a considerable portion being 
Fig. 39.—One-year old (1), two-year old 
(2), and. three-year old (3) shoots. In 
No. 3 the inner ring of bark is the. 
new bark, and the third ring of wood, 
counting from the pith, is the sap- 
wood (new wood), 
Fig.40.—One-year old stem, 
showing the pith and the 
pith-rays (silver grain). 
