290 PLANT LIFE. 



scales belonging to itself, and containing two minute 

 embryo needles. On looking back to the original older 

 branch, it is easy to find the traces of both the separate 

 and the general scales. 



The mature needles are hard and stiff. As regards 

 their internal structure there is one large vein in the 

 centre. This consists of two groups of xylein and 

 phloem connected with the sheath, or efidodennis ^ by 

 special transitional cells. The green cells are closely 

 packed together, and fill up the space between this 

 sheath and the layer of cells just below the epidermis. 

 The latter is made up of thick-walled cells which form 

 a supporting or strengthening cylinder, giving to the 

 leaf considerable rigidity. On looking for the stomata, 

 it will be found that they are sunk below the surface of 

 the leaf This arrangement hinders the read}^ escape 

 of water, so that there is but little transpiration ; and, 

 in fact, a pine gives off only about one-sixth part of the 

 water transpired by an average oak tree, or other 

 Dicotyledon, in the same time. This probably explains 

 why the needles are peculiarly subject to injury from 

 smoke or other impurities. In their natural position, 

 they are exposed to wind and snow, and are more likely 

 to lose too much than too little water by transpiration. 

 It often happens that, whilst the sun is shining, the 

 ground in a pinewood is frozen hard. Under these 

 conditions no water can enter the trees, and yet the 

 stomata will be open and will give off water vapour. 

 The result may be that the whole tree withers and dies 

 from lack of water. Thus there is a necessity for 

 keeping down transpiration, a necessity which the 

 arrangement of the stomata on the leaf seems intended 

 to meet. 



The leaves also show several conspicuous I'esin canals 



