396 LECTURE XVI. 



Since, as we see, light retards growth, it can be easily 

 conceived that its retarding effect will vary with its intensity. 

 Illustrations of this are afforded by variations in the size of 

 leaves grown under different conditions of illumination. Leaves 

 grown fully exposed to sunlight are smaller than those grown 

 in bright diffused light, and those grown in weak light are 

 small, not much larger, in fact, than those grown in darkness. 

 The most striking illustration is afforded by Wiesner's obser- 

 vation, to which allusion has already been made, that 

 exposure to very intense light arrests growth altogether. 

 The general relation of the intensity of light to the rate of 

 growth of shoots is then this, that at a certain maximum 

 intensity growth is prevented, and that, as the intensity di- 

 minishes, the rate of growth gradually increases, attaining a 

 maximum rapidity in darkness. 



The effect of light in retarding growth is not immediately 

 perceptible on exposure, at least in the case of organs of com- 

 plex structure ; nor does its effect immediately cease when 

 the organ is replaced into darkness. This is shewn in the 

 table of my observations on the leaf of Secale cereale (Rye). 

 During the period of exposure to light (11-12) the rate of growth 

 was somewhat retarded, and the retardation was probably 

 greater than it appears to have been, for the rate of growth 

 was increasing. But the full effect was exhibited in the suc- 

 ceeding hour (12-1) when, in spite of the natural acceleration 

 and of a rise of temperature, the rate of growth further 

 diminished, and it clearly persisted during the next hour. 

 We shall have occasion to consider the " persistent influence " 

 of light more fully when we are discussing the conditions of 

 the daily period of growth. 



We may now enquire whether or not all the rays of the 

 spectrum are alike capable of inducing a retardation of growth 

 in length. It may at once be stated that this is not the case. 

 The observations of Sachs, of Kraus, and of Rauwenhoff, shew 

 that when plants are grown in the rays of low refrangibility 

 (yellow, orange, red) they present, though not quite in the 

 same degree, the abnormalities of form which are characteristic 

 of etiolation, that is, their internodes become excessively 



