29.] 



CHAPTER II. THE TISSUES. 



131 



29. Forms of Tissue. According to the form and arrange- 

 ment of the constituent cells, the thickness and chemical compo- 

 sition of their walls, the nature of their contents, etc., it is possible 

 to classify the forms of tissue in various ways. 



Taking, first, the capacity for growth and cell-formation, 

 embryonic tissue or meristem is distinguishable from adult or 

 permanent tissue. The former consists of cells (e.g. in the growing- 

 point of a cellular plant) which grow and divide ; whereas the 

 latter consists of cells which have ceased to grow and divide, 

 having attained their definitive form and size ; and whilst meristem 

 consists entirely of true cells, permanent tissue may consist wholly 

 or in part of cells which have lost their cytoplasm. 



Taking, next, the form of the individual cells and the mode of 

 combination into a tissue which their 

 form determines, two forms of tissue 

 termed parenchyma and prosenchyma are 

 distinguished. In parenchymatous tis- 

 sue there is no great difference in the 

 three axes of the somewhat cubical cell, 

 and the cells are in contact by broad 

 flat surfaces (Figs. 86, 89). In prosen- 

 chymatous tissue, on the other hand, 

 (Figs. 90, 92), the cells are much longer 

 than they are broad, having pointed 

 ends which overlap and dovetail in 

 between those of other cells of the 

 tissue. 



By combining the distinctive characters which have just been 

 mentioned, with others which relate to the nature of the cell- 

 contents and to the constitution of the cell- wall and are intimately 

 connected with the functions of the cells, the following forms of 

 permanent tissue may be distinguished : 



1. Thin-walled parenchymatous tissue consists of cells having 

 cell- walls of cellulose. So long as the cells are functionally active 

 they contain cytoplasm ; they may eventually lose their cell-con- 

 tents and become dry and filled with air (e.g. pith of Elder). This 

 form of tissue is the main seat of the protoplasm in the plant, 

 and it is in the cells of this tissue that the chemical processes 

 connected with nutrition are more particularly carried on. It is 

 especially well-developed in fleshy and succulent parts (e.g. leaves, 

 fruits, tubers, tuberous roots, etc.). 



FIG. 90. Prosenchymatous 

 tissue, longitudinal section (dia- 

 gram, magnified), the pointed 

 ends of the elongated cells fit in 

 between each other. 



