THE MEASUREMENT OF GROWTH 



from the filaments of algae and fungi. So also are the oscillations due to variations 

 in the grand period of growth of successive internodes. These latter are also 

 autonomic changes, and afford as good an indication of the plant's power of 

 regulating its activity of growth, as does the sudden tearing of the cell-wall and 

 rapid elongation of the deposited ring of cellulose in a cell of Oedogonium which 

 is about to divide. 



The following table gives a series of measurements made by Hofmeister 1 

 upon Spirogyra princeps. The readings from the micrometer scale (i = 0-0028 mm.) 

 are given and also the increments of growth in scale divisions per minute. As can 

 be seen, growth continues slowly for some time, but suddenly is so accelerated 

 that a cell elongates by 7^ per cent, of its length in one minute. 



SECTION 6. The Measurement of Growth. 



In the case of rapidly growing plants the hourly growth can be 

 measured by means of an ordinary metre rule. By viewing through a 

 microscope the movement of the growing point can be directly seen, and 

 can be rendered visible to a large audience by means of a projection lantern. 



The Zeiss projection lantern permits of a magnification of from 10 to 10,000 

 diameters. The movement of swarm-spores and of streaming protoplasm can be 

 sufficiently magnified to become visible on the screen by using a powerful electric 

 light as the source of illumination, and passing the light through water and ferrous 

 sulphate solution to render it less injurious. For the demonstration of growth, the 

 first seedling leaf ofAvena or Hordeum is very suitable when 25 to 30 mm. high, and 

 it should be immersed in water in a glass vessel with parallel sides just before use. 

 The shadow of a small fixed rod serves to indicate the movement of the growing 

 object, which amounts to 60 mm. per minute, with a magnification of 4,000 when 

 the actual growth is 0-015 rnm. 2 



Hofmeister, 1. c., p. 222. 



a For details see Pfeffer, Jahrb. f. wiss. Bot., 1900, Bd. XXXV, p. 711. 

 C 2 



