THE STEM IQ 



are usually associated with broad leaves. From this it follows 

 that there is a correlation between the number of ranks of 

 leaves on the stem and the distance between leaf levels on the 

 one hand, and the form and size of the leaf on the other. 

 Plants which normally grow in tussocks, i. e., many stems in a 

 cluster, form a natural exception to the above rule, for in this 

 case the crowding of the stems reduces the number of leaves 

 possible on each stem. 



32. On horizontal branches the leaves are attached to the 

 stem in precisely the same order as on vertical stems, but the 

 blades of the leaves are in many cases brought round into the 

 horizontal plane by a twisting and bending of the petiole. 



33. Still other devices are employed for securing equal and 

 sufficient illumination of the leaves, but whatever the means 

 employed all tend toward the same result, viz., a maximum 

 exposure of green tissue to the light. 



The Stem 



34. From what has gone before, it is evident that one of 

 the functions of the stem of the plant is to hold up the leaves 

 to the Kght. This function may be performed in various ways, 

 and much of the character of the stem and its branches depends 

 upon how this function is performed. Among the low herba- 

 ceous forms the adaptation of the stem to this function is simple 

 enough and nothing further need be said here, except to note 

 that where the plants are crowded the stems are usually less 

 branched, and more slender than where they grow singly. 

 This is also true of trees, and the cause of it may be discovered 

 by the comparison of a few examples. 



35. A tree growing in an open space tends to have a relatively 

 short and thick trunk, with large, spreading branches near the 

 ground. One growing close by the side of another in an open 

 field will have the large branches only on the side away from 



