502 Minnesota Plant Life. 



Nevertheless the constant action of the force of gravity may 

 be said to induce the upright position of a normal shoot. It 

 is true this position may be modified in a variety of ways 

 by a variety of forces, or by the so-called correlations of growth. 

 A lateral ray of sunlight may force a shoot out of the perpen- 

 dicular, either towards the ray, as in the nasturtium, or away 

 from it, as in the ivy. And in an organ-complex, such as the 

 branching crown of a tree, the mutual interrelations of the twigs 

 cause positions to be taken which are quite the reverse of those 

 that they would take if growing by themselves. The young 

 sapling of the soft maple, for example, grows erect. When it 

 begins to branch the branches form angles with the primitive 

 stem, and, in a mature tree, twigs will be found growing directly 

 downward, perhaps a thing that they would not do unless 

 correlated with the other branches of the tree. Sometimes the 

 erect position of one organ inhibits such a position in a sub- 

 ordinated organ. An illustration of this is often seen in 

 pine trees. As long as the terminal shoot is active it is the only 

 one to maintain the erect position ; but let it be destroyed and 

 a 1'ateral shoot will bend into the erect position. Evidently the 

 lateral shoot has the structure fitted to maintain the erect posi- 

 tion and was prevented from so doing simply because correlated, 

 as a lateral branch, with the original terminal axis. 



When all necessary reservations have been made it becomes 

 apparent that most plant organs are strongly influenced by the 

 direction from which the forces of light and gravity may strike 

 them. Evidence of this may be seen whenever a grass stem 

 beaten down by hail lifts itself again, or in the leaning towards 

 the light, of plants in a window-garden. 



Besides the manifestations of irritability that have been 

 alluded to, there are many others, to which but a cursory refer- 

 ence can be made. Roots bend from a less moist to a more 

 moist substratum. They commonly bend away from the light. 

 Greater oiMess warmth may exert a directive influence upon 

 growth. Chemical substances in solution influence aquatic or- 

 gans. A number of organs grow towards oxygen or towards 

 the air. Others grow away from it. Thus, if several pollen- 

 spores germinate in a drop of sugar solution all the pollen- 

 tubes will grow towards the center of the drop, and away from 



