GROWTH, IRRITABILITY AND MOVEMENT 



145 



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is required. Thus we can measure increase in linear dimensions, in 

 volume, or in weight : the impression that is gained of the growth of 

 a plant or organ is, however, liable to vary according to the particular 

 index of growth that is adopted. The growth of a root or a stem 

 is usually measured by noting the increase in length over a given 

 time. For this purpose the horizontal microscope, of which different 

 patterns are available, may be employed, or one may use the anxano- 

 meter, in which the growth is magnified by means of levers or pulleys 

 for the purpose of measurement. 

 If the growth of the plant as a 

 whole is required it is usual to 

 observe the increase in its weight. 

 A special value as a growth- 

 index has been placed on the 

 dry weight of the plant, i.e. the 

 weight of matter remaining after 

 the water originally present has 

 been driven off by drying the 

 plant tissues in an oven. What 

 this method measures is the net 

 amount of environmental mater- 

 ial that has been incorporated 

 into the actual substance of the 



plant, irrespective Of whether Record of circumnutation in a seedling made by 

 • . , . , , . Malins Smith. Note that the direction was reversed 



this has been USed tor growth Or before 8.o a.m. in this record. 



deposited as a storage material. 



The determination of dry weight involves killing a plant, so that 

 consecutive observations cannot be made on a single plant : a series 

 of similar plants must be available when growth is to be measured 

 in this way. 



(c) Factors affecting Growth. 



Since in its growth the plant utilises the products of its assimilatory 

 processes, such as the synthesis of carbohydrate and of protein, it 

 is obvious that factors affecting those processes may also affect 

 growth. Thus growth will be hindered if adequate supplies of raw 

 materials such as carbon dioxide, water and nutrient salts are not 

 to hand : experiments indicating the importance of nutrient salts 

 have been described on p. III. Light must also be present if auto- 

 trophic growth is to occur, while oxygen is required for respiration. 



Some of these factors have specific effect on growth, and these have 

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Fig. 90. 



