48 



VEGETABLE PHYSIOLOGY. 



[LESSON 17. 



FiG.St 



solely due to the secretion of color-cells, or pigment, in a certain 

 layer of the skin ; the human hair, and the hair 

 of animals, depends for its color on a like se- 

 cretion within the cells of the pith and cortical 

 substance ; and the black, or choroid coat of the 

 eye, in man and animals, is solely composed of 

 minute cells, filled with infinitesimal particles 

 of a black paint. 



284. It is interesting to observe that the 

 same law is in operation to give color to the 

 leaf of a tree, and the skin, and other parts of 

 a man. 



285. It only remains to show the vascular 

 system of a leaf, and for this purpose the leaf 

 of a cherry tree is selected. Here the stalk 

 (petiole) ends in a single mid-rib (Fig. 84, a, a) ; 

 this gives oft primary veins (&), which subdi- 

 vide into secondary veins (c), curving within 

 the margin. 



Clierry leaf. 



LESSON XVII. 



OF THE STEMS OF TEEES. 



286. The anatomical character of the stems of trees must now be 

 considered. This structure consists of the elementary tissues, vari- 

 ously combined, and arranged in different ways. 



287. In some plants the part which represents the stem is entire- 

 ly composed of cells, which take the form of very narrow filaments ; 

 they are either simple or branched, as in some of the fungi and con- 

 fervse, or they form an expanded thallus, or frond (a term applied to 

 the stem of certain plants, where the stalk and leaves are so inti- 

 mately blended that they cannot be separated). In well formed, 

 conspicuous stems, cellular and vascular tissue are both present. 



288. Such stems always have as the basis of their structure a 

 dense cellular parenchyma, in the midst of which is usually found 

 fibro-vascular bundles, or fasciculi of woody fibres, with ducts of va- 

 rious kinds, and generally associated with spiral vessels. 



289. It is in the mode of arrangement of these bundles, that the 



