46 NATURE TEACHING. 



STRUCTURE OF STEMS. 



1. A piece of the stem of a cacao, mango 

 hibiscus, rose or other tree, when cut across 

 and examined is seen to be composed of vari- 

 ous 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 firm 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 ; outside 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 hibiscus), an 

 inner fibrous layer, a middle, thick, 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. 



2. 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 



