Power to Adjust Parts to Surroundings 



227 



tropism (pronounced with the accent on the second syllable), or 

 Heliotropism. Parts that turn towards light are described as 

 positively phototropic (with the accent, despite the seeming 

 anomaly, on the third syllable), those that turn away as negatively 

 phototropic, and those that turn 

 across as transversely phototropic. 

 Phototropism is so thoroughly typi- 

 cal an example of the power of indi- 

 vidual plant parts to adjust them- 

 selves in relation to the immediate 

 external conditions that we can use 

 it as a basis for the analysis of the 

 nature of this power, which is known 

 physiologically, though not very 

 happily, as Irritability. Now the 

 elements entering into irritable re- 

 sponses are these : — 



First, the reason why the parts do 

 it. — As to this, the explanation 

 must be amply obvious. The turn- 

 ing towards the window brings the 

 leaves into positions where they 

 secure the best exposure to light, — 

 the light which is indispensable to the photosynthetic func- 

 tion for which they exist. The best position for performance 

 of this function must of course be that which sets tliem at 

 right angles to the light; and this in turn requires that the 

 stem, whose function is simply to carry the leaves, shall point 

 or reach towards the light. As to the roots, not only does 

 their function (the absorption of water and minerals), require 

 no light, but their unprotected protoplasm is actually injured by 

 exposure thereto ; and this shows the advantage of their power to 

 retreat from light. The reason for the characteristic phototropism 

 of ordinary leaves, stems, and roots, respectively, is therefore to 



Fig. 76. — A Mustard seedling germi- 

 nated by water culture in darkness 

 and then exposed to light falling 

 from the direction of the arrow. 



