46 NATURE TEACHING 



rhizome and the potato tuber ; and finally we get the 

 crocus corm, a very compact, stem structure containing 

 food-leaf and flower-buds, with a protective covering. 



Structure of Stems. 



A piece of the stem of a horse chestnut, elm, oak, ash, 

 hawthorn, rose, or other tree, when cut across and ex- 

 amined, is seen to be composed of various parts arranged 

 in a definite manner. In the middle there is a soft 

 portion, the pith, small in some cases, large in others ; 

 this is surrounded by hard wood, which, in the case of 

 old trees, makes up the greater portion of the stem, 

 whilst in young branches it only forms a thin ring ; out- 

 side of all is the bark, sharply marked off and easily 

 separable from the wood. The bark itself is made up 

 of three layers (easily recognised in the horse chestnut 

 or ash) an inner, fibrous layer; a middle, green portion; 

 and an outer, thin brown layer, not at all fibrous, but 

 which readily breaks in pieces if any attempt is made 

 to detach it. 



The region where wood and bark join is of great 

 importance, for there is present, between these two 

 conspicuous tissues, a soft, somewhat slimy, thin layer, 

 best seen in young, vigorously-growing shoots. This 

 layer is the cambium, or growing layer, and consists of 

 young growing tissue similar to that which is present at 

 the apices of stems and roots. The cambium has the 

 power of producing new tissue in either direction ; that 

 is to say, situated as it is between wood and bark, it can 

 add both to the wood and to the inner bark. The 

 increase in thickness of the wood is generally very much 



