IO Practical Plant Biology. 



can be easily measured by using the walls of other cells more 

 suitable for the purpose. Thus the fibres of cotton-wool are the 

 walls of very long narrow cells in which the internal cavity of the 

 cell has been almost obliterated by the thickening of the wall. It 

 is possible by measuring the weight, which one of these fibres will 

 sustain, and also its own cross section, to determine directly the 

 tenacity of its substance. It has been found that the breaking 

 weight of this material is about 4*2 tonnes per sq. cm. This is 

 about the same tenacity as that of wrought iron. 



Our preliminary observations have brought to light yet another 

 very important property of this material. When the liquor iodi 

 was applied to the outside of the cell-wall, it quickly diffused 

 through the wall and coloured the nucleus within. Evidently the 

 cell- wall is permeable to this substance dissolved in water. It is 

 in fact permeable to all substances dissolved in water. 



Chemically the substance forming the cell-wall is known as 

 cellulose. It is a carbohydrate, i.e. a compound containing carbon, 

 hydrogen and oxygen, in the molecule of which there are two 

 hydrogen atoms to each oxygen atom. It has a large and com- 

 plicated molecule, having the formula (C 6 H 10 O 5 ) n . 



The protoplasm enclosed by the cell-wall is much less con- 

 spicuous than the wall itself ; but this does not indicate by any 

 means that it is of less importance than the wall. Indeed, as will 

 appear later, it forms the essential part of the cell. 



By similar observations to those made on the cell-wall, we can 

 easily ascertain that protoplasm is colourless, transparent and of 

 a higher refractive index than water ; but in this cell its refractive 

 index is not so high as that of the cell- wall. Where the wall of a 

 cell is broken the protoplasm emerges, and it may be seen to 

 change shape slowly, like a sticky, viscid liquid. It adheres to 

 the fragments of the cell-walls and it shows no rigidity or elasticity 

 like cellulose. In fact, it behaves very much like the white of an 

 egg, and in the cell we are examining the protoplasm forms a 

 sticky, viscid lining to the cell-wall, which might be compared to 

 a smear of white of egg on the inner surface of a transparent box. 

 In this viscid film are often embedded minute granules ; some- 

 times again the granules are absent, and the protoplasm seems 

 homogeneous or almost so. By closing the diaphragm it is 

 generally possible to make out a definite surface between the proto- 

 plasm and the water in the vacuole. 



The protoplasmic lining of the cell-wall encloses the liquid of 

 the vacuole. This liquid is a watery solution of various sub- 

 stances. Unlike the cell-wall the protoplasm obstructs the passage 



