26 



PLANT STRUCTURE AND GROWTH. [CHAP. 



FIG. 



FIG. 26. 



sure of neighbouring cells. It comprises several varie- 

 ties, of which the most important is Parenchyma, 



in which the cells are so 

 arranged as to leave spa- 

 ces (intercellular spaces], 

 through which air can cir- 

 culate between the cells. 

 It forms the principal por- 

 tion of most plants, the 

 other tissues being- embedded in it. 



o 



Another form of cellular tissue is called Scleren- 

 chyma, from the cells being hard, owing to the thick- 

 ening of the cell-wall. The " stones " of cherries, 

 plums, &c., are composed of sclerenchyma. 



Some of the cells assume a long, narrow, drawn-out 

 form, like fibres or tubes, and from their appearing in 

 compact masses among the parenchyma, they are 



known as fibro - vascular 

 btindles. It is these bundles 

 which form the midrib and 

 " veins" of leaves and the 

 hard part of the stem. 

 They are made up of wood- 

 tissue, bast-tissue, and vas- 

 cular tissue, packed together 

 in wedge-shaped masses 

 (fig. 27). On examining a cross section of one of 

 these wedges with the microscope, we find it pre- 

 sents the appearance of fig. 28. o is the outer, i the 

 inner end ; B is the bast-tissue, C the cambium zone, 

 W wood-cells, and V the openings of the spiral 

 vessels and ducts. 



FIG. 27. 



