26 PLANT STRUCTURE AND GROWTH. [cHapP., 
sure of neighbouring cells. It comprises several varie- 
ties, of which the most important is Pavrenchyma, 
in which the cells are so 
arranged as to leave spa- 
ces (zntercellular 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. 
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 
bundles. Itis 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, z 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. 
Fic. 25. 
