STRUCTURE OF STEMS 65 



show the same arrangement of the cells, and will give an idea of 

 the length of the different elements, and show the markings of 

 the vessels and the character of the sieve tubes. 



105. Structure in cross section of perennial woody stems. 

 A cross section of a stem several years old will show the follow- 

 ing structure. In trees like the oak the chief points in the 

 structure can be seen readily with the eye or with the aid of 

 a hand or pocket lens. The character of the "bark" will depend 

 on the age and the kind of the tree. If the stem is only a few 

 years old the bark will be green and soft. This soft bark is made 

 up of the bast portion of numerous nbro-vascular bundles lying 

 side by side, and in it are the sieve tubes. On older stems the 

 outer bark is dead and often cracked into deep furrows. This 

 is the true bark. It is formed by a layer of cells on the outside 

 of the bast portion called cork producer. The soft "bark" lies 

 underneath the coarse dead corky bark in the old stems. In the 

 spring, the soft bark can be very easily stripped off from the 

 stems, as in the willow, basswood, etc. The tissue where the 

 bark parts from the stem when stripped off in this way is the 

 young and delicate cambium, which, we found in paragraph 104, 

 forms a continuous layer entirely around the stem between the 

 bast and the wood of the bundle. The portion of the stem lying 

 inside of this layer then is the wood, except the central portion 

 or pith.* The wood portion of old trees consists of a whitish 

 outer portion called the sap wood, while the darker inner portion 

 is the heart wood. The heart wood is dead, but in the sap wood 

 there are many living cells and it is here that the rise of water in 

 the tree takes place. No rise of water takes place in the dead 

 heartwood. There are three peculiarities of the woody portion 

 of such a stem which are visible to the eye. First, the slender 

 whitish lines which radiate from or near the pith to the outside. 

 These are the medullary rays or pith rays which (paragraph 102) 

 lie between the nbro-vascular bundles. They consist of paren- 



* The pith varies greatly in extent in different trees and shrubs. In 

 some it is very abundant, as in the elder, sumac. It may be continuous, 

 chambered or diaphragmed, etc., in different species. 

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