lESSON 23.] CELLULAR TISSUE. 143 



make the gi-owth of the plant, as was shown in the last Lesson. 

 We cannot divide them into similar smaller parts having the prop- 

 erties of the whole, as we may any mineral body. We may cut 

 them in pieces ; but the pieces are only mutilated parts of a cell. 

 This is a peculiarity of organic things (2, 3) : it is organic structure. 

 Being composed of cells, the main structure of plants is called 



393. Cellalar Tissue. The cells, as they multiply, build up the 

 tissues or fabric of the plant, which, as we have said (389), may be 

 hkened to a wall or an edifice built of bricks, or still better to a 

 honeycomb composed of ranges of cells (Fig. 340). 



394. The walls of the cells are united where they touch each 

 other ; and so the partition appears to be a simple membrane, 

 although it is really double ; as may be shown by boiling the tissue 

 a few minutes and then pulling the parts asunder. And in soft fruits 

 the cells separate in ripening, although they were perfectly united 

 into a tissue, when green, like that of Fig. 340. 



395. In that figure the cells fit together perfectly, leaving no 

 interstices, except a very small space at some of the corners. 

 But in most leaves, the cells are loosely heaped together, leaving 

 spaces or passages of all sifes (Fig. 356) ; and in the leaves and 

 stems of aquatic and marsh plants, in particular, the cells are built 

 up into narrow partitions, which form the sides of large and regular 

 canals or passages (as shown in Fig. 341). These passages form 

 the holes or cavities so conspicuous on cutting across any of these 

 plants, and which are always filled with air. They may be likened 

 to a stack of chimneys, built up of cells in place of bricks. 



396. When small and irregular, the interstices are called inter- 

 cellular spaces (that is, spaces between the cells). When large and 

 regular, they are named intercellular passages or air-passages. 



397. It will be noticed that in slices of the root, stem, or any tissue 

 where the cells are not partly separate, the boundaries of the cells 

 are usually more or less six-sided, like the cells of a honeycomb ; 

 and this is apt to be the case in whatever direction the slice is made, 

 whether crosswise, lengthwise, or obliquely. The reason of this is 

 easy to see. The natural figure of the cell is globular. Cells which 

 are not pressed upon by others are genei-ally round or roundish 

 (except when they grow in some particular direction), as we see in 

 the green pulp of many leaves. When a quantity of spheres (such, 

 for instance, as a pile of cannon-balls) are heaped up, each one in the 

 interior of the heap is touched by twelve others. If the spheres be 



