12 VEGETABLE PHYSIOLOGY. [LESSON 4. 



when first developed, but the elasticity of the membrane composing 

 their cell-wall, readily yields to the pressure of surrounding like 

 cells, by which they assume an egg-shape or elongated form (Fig. 

 10); they are often angular (Fig. 11), or even polyhedral (many 

 sides) (Fig. 12). 



FIG. 10. FIG. 11. FIG. 12. 



Single cell of Pineapple. Cells of Cucumber. Polyhedral cells. 



67. Again, the size of cells is found to be as variable as their 

 figure, in different plants, and in different parts of the same plant. 



Dr. Hook counted more than a thousand in a line, an inch long, 

 in Cork ! 



68. Notwithstanding each cell originally consists of a separate 

 membrane, the walls of contiguous cells may become united in the 

 progress of growth. 



69. When cells are united at their extremities, it frequently hap- 

 pens that the walls, which form the point of junction) become ab- 

 sorbed, and thus a tube is formed. 



70. The simplest kinds of plants, as Sea-weeds, are called cellu- 

 lares, being entirely composed of cellular tissue ; so too the pulpy 

 parts of fruits and vegetables, and it is the great object of Horticul- 

 ture to extend, as far as possible, the production of this tissue. 



71. The pith of trees and plants is cellular ; and such is the 

 structure of cotton and rice-paper. 



72. Cells generally transmit fluids through the parietes of the 

 cell-wall, to which rule some mosses and other plants offer exception ; 

 in them, minute apertures, or holes, are found in the cells in a sin- 

 gle cell of a Euphorbiaceous plant, as many as forty-five openings 

 have been counted. 



73. Another form of cell is known as tne porous cell, in which 

 the membrane has been thickened at certain parts, leaving thin round 

 spots, which, viewed by transmitted light,* appear like holes (see Fig. 



* There are two modes of illuminating objects seen by the microscope ; one is, to throw 

 light upon the object this is called direct light. The other plan consists in throwing light 

 through a transparent object this is called transmitted light. 



