194 TEXTBOOK OF BOTANY 



213. Influence of Gravity and other Stimuli. Stems that 

 normally grow erect are affected also by the stimulus of grav- 

 ity, but in a way opposite to that in which the same stimulus 

 affects roots. This is shown by stems growing in darkness, 

 like the shoots that grow from potato tubers which are stored 

 in a damp, dark cellar. Such a shoot, even if in absolute 

 darkness, still tends to grow upward, that is, in a direction 

 opposite to that of the force of gravity. But its upward growth 

 is often hindered by the fact that it is too slender and weak 

 to hold itself erect, and so it may be only the growing end 

 that turns upward. Ordinarily, the influence of light arid 

 that of gravity work in the same direction, and combine to 

 induce stems to grow erect. But sometimes, as in the case 

 of house plants standing near a window, these two influences 

 work in different directions. In such a case the influence 

 of light is usually stronger than that of gravity, and the plant 

 becomes one-sided. The stimuli of light and gravity are 

 not the only ones that affect the growth of erect stems and 

 branches. It is probable that many other stimuli, such as 

 moisture or dryness, air pressure, and changes of tempera- 

 ture, are of importance in connection with growth. Some of 

 them certainly affect the rate of growth. But the effects of 

 these and of other factors upon the direction of growth are 

 not easy to study, because their influence is small in com- 

 parison with that of light and gravity. 



214. Direction of Growth of Branches. As a rule, the 

 branches that come from an upright stem do not grow straight 

 up ; they grow either horizontally or more or less diagonally 

 upward. The branches, like the main stem, are influenced 

 in their growth by the stimuli of light and gravity. Probably 

 most branches tend to grow toward the light. But since a 

 branch is shaded more or less by the main stem and its leaves, 

 as well as perhaps by other branches and their leaves, the 

 stimulus of light alone would ordinarily cause it to grow, not 

 directly upward, but in that direction from which it receives 



