48 VEGETABLE HISTOLOGY. 



CHAPTER VIII. 

 THE TISSUES OF THE HIGHER PLANTS. 



The lessons thus far have been given to the study of some of 

 the simple plants, for the purpose of giving an idea of the 

 nature of the lowest forms of plant life as well as familiarizing 

 the student with the use of the microscope and the manipula- 

 tion of objects on the slide. Some of the plants studied play an 

 important role in the life economy, for example, yeast, bacteria, 

 moulds, and thus deserve close study. While studying these 

 plants we have learned what is meant by a plant cell, and the 

 subsequent lessons w^ill be devoted to a study of the various 

 kinds of cells and webs of cells known as "tissues," found in 

 the most highly developed and complex plants, the Phane- 

 rogamia or flowering plants. 



The peculiarity of these is that there is a great division of 

 labor, with corresponding tissues and organs, which have been 

 differentiated from a fundamental tissue. Thus we have a leaf, 

 an organ for manufacturing protoplasm and starch ; the flower, 

 which is the reproductive organ; the stem, the channel for 

 conveying sap; the roots for imbibing water and nourishment. 

 The cells are differentiated into distinct tissues. On passing 

 down to the lower series of plants these tissues become simpler 

 until, finally, in the Thallophyta and most of the Bryophyta we 

 have no distinction of tissues at all. Those plants consist of 

 a homogeneous mass of cells, as we have seen in the case of 

 algje and moulds. 



The various tissues or cell-webs of the flowering plants, 

 namely, epidermal, ground, fibro-vascular, stony, etc., are all 

 derived from cells that were at one time all alike. By various 

 physical and chemical modifications the cells come to differ 

 from one another and thus to give rise to the different tissues. 

 The cells of stony tissue, as found in shells of nuts, were once 

 like the soft cells of a leaf, but they became subsequently hard- 

 ened and modified. 



A cell has been defined as a nucleated mass of protoplasm. 

 It may or may not possess a cell-wall of different composition. 

 With rare exceptions in vegetable cells such a wall is present, 

 while most animal cells are destitute of it ; but in all essential 

 respects animal and vegetable cells resemble each other. Cells 

 are the structural units of the organism. All plant bodies are 

 composed of cells or of these together with the products of cell 

 activity. Within the compass of the cell occur all those essen- 

 tial phenomena which are called vital ; the life of a plant resides 

 in its cells; the sum of the activities it exhibits is the sum of 

 the activities of its component cells. 



