290 BOTANY PART i 



Comparison of an etiolated plant with one grown in the light shows 

 that the influence of light is not the same on all organs ; it may 

 either increase or arrest the growth. While, however, the action 

 of light in arresting the growth of the stem increases with the 

 intensity, the increase of the growth of leaves due to the light has 

 a limit ; the leaf attains its maximal size in light of moderate intensity. 

 It is a one-sided view of the growth in length of the stem, and the 

 resulting height .of the plant that is expressed by the statement, 

 "the effect of illumination is to retard growth." In these organs, 

 and in others that behave similarly, the effect of light is found 

 to be much less simple. It appears rather, as is shown by the 

 accurately -investigated case of the coleoptile of Avena, that light 

 first accelerates and then retards growth, and that both influences 

 increase with the intensity of the light. With every increase in 

 the illumination there is first acceleration and then retardation of 

 growth, while on darkening the plant there is retardation followed by 

 acceleration ( 57 ). 



The effect of the component rays of white light appears to be still 

 less simple. When light is arresting the elongation of the stem it is 

 the blue and violet rays of short wave-length that are effective, while 

 the red rays behave in the same way as darkness. Other processes 

 of growth, however, are influenced differently. The germination of 

 the spores of certain ferns is accelerated by red light, while blue 

 light hinders it even more than darkness. Spores germinated in red 

 light produce greatly elongated cells which only become divided by cell 

 walls in blue light ( 57a ). The complicated nature of the phenomena 

 is in part explained by light acting both as a stimulus to growth and 

 as a source of energy. Ultra-violet light injures the plant ; radium- 

 and Rontgen-rays retard, but, like poisons (p. 29 4), may when in 

 small quantity promote growth ( 58 ). 



In addition to the intensity and the quality of the light, its 

 direction greatly influences the form of the plant body. The 

 curvatures due to one-sided illumination (phototropism) will be dealt 

 with later in connection with the phenomena of movement. The 

 illumination may also influence the polarity and symmetry of the 

 plant. Thus in some simply-organised plants the more strongly 

 illuminated side of the cell from which development starts becomes 

 the apex and the other side the base. In other cases an originally 

 radial growing point becomes bilateral or dorsiventral under one- 

 sided ^Humiliation. Lastly, an organ which has passed the embryonic 

 stage may become dorsiventral, as in cases where roots form on the 

 shaded side only. When it is possible to experimentally transform 

 the external symmetry the internal structure is also as a rule altered, 

 the connection between the two being very close. 



In the germination of the spores of Equisetum, the first division wall, and with 

 this the distinction of apex and base, is determined by the direction of the light. 



