2] 



UPOX THE DIRECTIOX OF GROWTH 



437 



2. EFFECT OF LIGHT UPON THE DIRECTION OF GROWTH 



-PHOTOTROPISM * 



Under this topic will be considered, first, the effect upon 

 plants ; secondly, upon animals ; and, after that, certain gen- 

 eral matters concerning phototropism. 



1. Plants. The fact that seedlings reared in a room near a 

 window all have their tops directed towards the source of light 

 instead of vertically can 

 scarcely have escaped any 

 one's notice. References to 

 the phenomenon are found 

 in the literature of the an- 

 cients ; its scientific study 

 was begun in the early part 

 of the last century by HALES 

 (1727). Not only the stems 

 of seedlings but many other 

 plant-organs show this 

 growth with reference to 

 the direction of the infalling 

 rays of light. Among these 

 are tips of many stems, many 

 leaves, cotyledons, roots (es- 

 pecially aerial ones), tendrils, 

 the fruit-bearing hyphse of 

 cryptogams, and certain or- 

 gans of the bryophytes and 

 pteridophytes. 



The sense of the turning 



in ordinary daylight is not FIG. 124. -Seedling of Sinapis alba exhibiting 



J J positive phototropism of the stem, ab, and 



always the Same. While the negative phototropism of the root, de. nn, 



Stems of most seedlings of surface of the water in which the plant is 



, germinating. The arrow indicates the di- 



phanerogams turn towards rection of the illfalling light rays . (From 

 the light (positive photo- FRANK, '92.) 



* On some accounts it is unfortunate to accept this word rather than the 

 older, more familiar term " heliotropism "; but as the latter is obviously unfitted 

 to our broader view of the subject, and encourages the introduction of new 

 special terms, such as selenetropisrn or turning towards the moon (MussET, '90), 

 I think it is desirable to adopt the newer term. 



d 



