72 NATURE TEACHING. 



may be taken as indicating to which of these 

 two groups a plant belongs. 



3. It is impossible, without the use of a 

 microscope, to obtain very much information 

 concerning the internal structure of leaves. 

 If, however, we select some thick-leaved plant, 

 such as the Spanish 'needle' or the 'dagger,' 

 we find that both upper and lower surfaces of a 

 leaf are covered with a colourless skin which, 

 with a little care, can be stripped off. The 

 main mass of the leaf is seen to be made up 

 of a rather soft, more or less spongy tissue 

 through which fibrous strands the veins- 

 run. The thin skin makes a kind of water- 

 proof coating to the leaves, but has an 

 enormous number, of minute openings, called 

 stomata (too small to be seen without a mag- 

 nifying glass) through which the gases of the 

 atmosphere can pass in and out and so reach 

 the spongy tissue of the inside of the leaf. 

 This is most important, for it is in this inner 

 part that the real work of the leaf, the breath- 

 ing and building up of new matter goes on, 

 and, for these processes, a free interchange 

 of gases with the outside air is absolutely 

 necessary. 



