6;6 



SCIENTIFIC RECREATIONS. 



obviously distinct from each other ; and as one year is requisite for the 

 formation of a single layer of wood, these depositions are named annual 

 layers or rings. In fig. 759 we have a representation of a stem three years 

 old, and one of five years of age. 



The number of rings in the stem do not invariably agree with the 

 number of years the tree has been growing, but it may be accepted as a 

 rule. 



The stem is the medium of communication between the roots and 

 leaves at first ; but after a year this important duty is deputed to the 

 cambium layers of new woody tissue, etc., and as time goes on the living 

 power has accumulated immediately under the bark. So although the tree 

 be quite hollow it will live. The interior has been closed up by deposition 

 of wood and has decayed ; but the life functions being relegated to the bark, 

 the old tree lives on. If we remove the rind all round a tree it will die. 



THE LEAVES. 



When in the spring the young leaves appear upon the trees, and as 



summer advances they become fully 

 developed, we are all grateful for the 

 beautifully varied tints of green, and for 

 the shade we can so fully enjoy. The 

 study of leaves is a most interesting and 

 instructive one, and nobody should omit 

 to examine them. Their forms are in- 

 finite, or, at any rate, countless ; their size 

 as varied as their forms. Many attributes 

 of the leaf will occur to every reader, and 

 we will briefly describe these essential 



organs of plants. Air and light are necessary to the development of leaves, 



and their principal use is to present a surface to the food material which the plant 



absorbs. They breathe, as it were, and absorb the 



carbonic acid from the atmosphere. These functions 



are called " assimilation," " transpiration," and " re- 

 spiration," which we will detail by-and-by. 



Leaves are distinguished according to position 



and duration. Some leaves have very simple forms, 



others are compound, so to speak. Some are plain 



and rounded, others are toothed, like the holly. The 



skeleton of a leaf is a very interesting study, and it 



will show the beautiful structure of these common 



objects. The delicate lines of the green leaf are 



"veins," or sap-vessels, which convey the necessary Fig. 7 6i. -simple leaf. 



nourishment. The leaves are called the " embryonic " (seed-like), the 



radicle, or root-leaf, the stalk-leaf, and the stipules, which grow at the base 



of stem-leaves, and the floral leaves, which bear the flowers or fruit buds. 



