CELL COMMUNITIES: A CHAPTER ON TISSUES 



85 



the veins. The outer skin consists of a single layer of cells and is called 

 the epidermis (Greek epi, upon, and derma, skin). The veins are composed 

 in the main of bundles of vessels and long woody cells, and belong to what 

 is called the fibro-vascular system ; and the soft tissue which constitutes the 

 rest of the leaf is known as fundamental or ground tissue. 



Here, then, we have the three great divisions under which all permanent 

 tissues and vessels naturally fall. Let us go over them again. First, there 

 is the epidermis, a thin cellular covering on the exterior of the plant; 

 secondly, the fibro-vascular system, consisting chiefly of wood-cells and 

 vessels, united in bundles, which extend from the roots to the leaves and 

 really form the skeleton or framework of the plant ; and thirdly, the funda- 

 mental or ground tissue, which occupies most of the space in the young 

 plant and consists chiefly of parenchyma. The lower plants that is, the 

 Fungi, Algce, Liverworts (Hepaticce). and Lichens have 110 fibro-vascular 

 bundles, and the Mosses (Musci) only contain them in a very rudimentary 

 form. Plants belonging to 

 the Cone-bearing order 

 (ConifercB), as the Pines, 

 Larches, Yews, and Cedars 

 (Pinus, Larix, Taxus, Ce- 

 drus), have wood-cells, but 

 no true vessels, their place 

 being taken by tracheides, 

 which are not continuously 

 open. In the higher plants, 

 the woodrcells of the fibro- 

 vascular bundles serve as 

 the channels by which the 

 crude sap, which holds in 

 solution the nutritious prin- 

 ciples, is conveyed from the 

 soil to the leaves. The ves- 

 sels are charged with air. 



Before leaving this part 

 of our subject we ought to 

 mention that beneath the 

 single layer of epidermal 

 cells we have in the leaves 

 a layer of elongated cells 

 packed closely side by side 

 in a direction vertical to the 

 surface of the leaf, and these 

 are known as palisade cells, 

 and in the aggregate as 



IE. Step. 



FIG. 117. LICHEN ON HOLLY BAKK. 



This Lichen (Graphis elegans) consists of raised dark lines on the smooth 



bark of the Holly-tree, which look much like the characters of some 



Oriental alphabet. 



