90 PART II. ANATOMY AND HISTOLOGY. [ 28 



cavities extending through whole internodes of many herbaceous 

 stems (Grasses, Umbelliferae, Equisetacese), and those occiirring in 

 leaves (Leek). 



The intercellular spaces which contain certain peculiar sub- 

 stances are treated of under the head of Glandular Tissue in 28. 



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

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

 tion 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 protoplasm. 



Taking, next, the form of the indi- 

 vidual cells and the mode of combination 

 into a tissue which their form determines, 

 two forms of tissue termed parenchyma 

 and proscnchyma are distinguished. In 

 parenchymatous tissue there_Js^ no great 

 difference in the three axesjrf_jjie_sojne- 

 what cubical cells, and the_ ells_are in 

 contact by broad flat surfaces (Figs. 60, 

 68). In prosenchymatous tissue, on the other hand (Figs. 68, 70/), 

 the cells are much longer than they are broad, having pointed 

 ends which overlap and dovetail in between those of other cells of 

 tli<! 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 protoplasm ; they may eventually lose their cell-con- 

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



ii 



FIG. 63. Prosenchymatous 

 tissue, longitudinal section (dia- 

 gram, magnified), the pointed 

 ends of the elongated cells fit in 

 between each other. 



