1 6 Practical Plant Biology. 



found that it becomes permeable to dissolved substances. Thus 

 if cells containing a coloured solution in their vacuoles be heated 

 to 70 C. the coloured fluid permeates the envelope and escapes. 

 At the same time the tissue which had been turgid and stiff 

 becomes limp and flaccid, as the osmotic pressure of its com- 

 ponent cells is relieved. 



At the temperature of about 70 C. protoplasm sets, i.e. it 

 changes from being a liquid to take on a semi-solid state. At 

 the same time it becomes more opaque. This change is called 

 coagulation and is very similar to the change undergone by the 

 white of an egg when it is boiled. 



With regard to the chemical nature of protoplasm very little 

 can be said with certainty. It appears to be a mixture of a 

 number of different proteins, lipoids, carbohydrates and salts. 

 The ingredients of different samples vary, and, owing to this 

 variability, it is not known whether there is some essential con- 

 stituent of constant composition in which the other compounds 

 are embedded in varying proportions. Not only do the ingredi- 

 ents of different samples vary, but within the same sample they 

 are constantly undergoing chemical change. These interactions 

 going on within protoplasm are spoken of as its metabolism. 

 Ultimate analyses of protoplasm always show the presence of the 

 following elements : carbon, hydrogen, oxygen, nitrogen, sulphur, 

 phosphorus, potassium, magnesium and calcium. The proportion 

 of these elements depends on the proportions of their compounds 

 in the mixture. 



Under normal conditions the protoplasm of cells contains a 

 very large proportion of water : 80 per cent, by weight, or more, 

 is usually present. Proteins rank second by weight, and it is 

 probably these substances which confer on protoplasm its unique 

 properties. These bodies may be defined as nitrogenous sub- 

 stances, which on hydrolysis break up into amino-acids, i.e. 

 organic acids characterised by containing one or more amino- 

 groups ( NHJ and one or more carboxyl groups ( COOH). 

 These groups confer on the amino-acids basic or acid qualities 

 depending on the predominance of either group. Consequently 

 they may behave as weak bases or acids. The amino-acids also 

 may combine mutually by the elimination of water from the uniting 

 carboxyl and amino-groups e.g. : 



CO pH H| NH- 



In this way many amino-acids may link together and bodies with 



