73 



CHAPTER II. TISSUES. 



68. A single cell which continues Different 



to grow and divide will result in producing an aggregate of K . inds of 

 cells all of which are coherent from the time of their origin; and^heii 

 such an aggregate of cells which are more or less similar, which Elements, 

 erow in the same way and which have similar duties to per- 

 form is called a tissue. As the cells of a tissue grow older their 

 walls frequently separate somewhat from one another and 

 spaces arise between them which are called intercellular spaces. 

 The tissues of plants may be first classified as : 



(A) Meristematic, or embryonic. Those consisting of cells 



still capable of growing and dividing and thus of 

 producing new cells. 



(B) Permanent. Those consisting of cells which have com- 



pleted their growth and have ceased to divide. 



The cells of permanent tissues vary greatly in size, shape, 

 thickness of their walls, and in other particulars. They may 

 remain more or less rectangular, or cubical, in shape, and a 

 tissue consisting of such cells is named parenchyma. The cell 

 walls are usually thin and the cells usually contain protoplasm. 

 Sometimes the cells have their walls thickened, but the thick- 

 ening substance is situated mainly at the corners of the cells, 

 in which case the tissue is called collenchyma. In other cases 

 the cells become much elongated in proportion to their width 

 and their ends are pointed. They then usually contain no 

 protoplasm but only air and water, while their walls are 

 generally thickened and provided with simple pits. Such cells 

 are called fibres. Cells, whether parenchymatous, or fibrous, 

 which have their walls very much thickened and in which the 

 cell-cavity is consequently very small and in some cases almost 

 obliterated, are said to be sclerenchymatous, and together form 

 the tissue known as sclerencltyma. Cells similar to fibres but 

 which are somewhat shorter and wider, with blunter ends, are 

 called tracheids. 



By the absorption of their transverse partitions, also, rows of 

 cells may become converted into long tubes which are termed 

 vessels, or tracheae. Both vessels and tracheids frequently 

 have typical bordered pits on their walls and they are also 

 distinguished as annular, spiral, or reticulate, when the 

 thickening bands on their walls form distinct rings, spirals, or 

 a net work, respectively. When the bordered pits of a vessel 



