336 Vegetable Physiology. 



axis. As the branch grows, the insertions of the leaves, which 

 were at first close together, become separated by the lengthen- 

 ing of the branch ; and they then usually assume something of 

 a spiral arrangement around it. This may be regarded as the 

 regular mode in which leaves are arranged. Starting from any 

 one leaf, we shall generally find the next leaf not exactly 

 above or below it, but alittle on one side of the perpendicular* 

 the next a little to one side of the last, and so on. The num- 

 ber of leaves taken to complete the spiral varies in different 

 plants. Sometimes it amounts to twenty or more. Sometimes 

 we find only two, in which case they are nearly on opposite 

 sides of the stem, but one is higher up than the other. Such 

 leaves are said to be alternate. The point of the stem at which 

 the leaf originates, is termed a node, and the space between 

 two nodes is an internode. 



Now although it is considered the regular mode of growth 

 for a branch or stem to lengthen equally throughout, yet many 

 varieties in the arrangement of leaves are met with, occasioned 

 by the cessation of growth at particular points. Thus, if the 

 internode between any two alternate leaves is not developed, 

 they will be opposite to each other. Again, where each spiral 

 turn contains several leaves, if all the internodes between the 

 highest and the lowest be undeveloped, these leaves will arise 

 from the same point of the stem, still growing, however, in 

 their proper directions ; so that a complete circle of leaves, 

 resembling that of the leafy parts of a regular flower, will be 

 produced. This is called a whorl or verticil. There are some 

 plants which exhibit the true spiral arrangement, as their regu- 

 lar mode of growth ; others in which the leaves are constantly 

 opposite ; and in some they are always verticillate. 



But there are many species which present differences in 

 arrangement in the same individual, according to the circum- 

 stances under which each part has been developed ; and by 

 such examples the connexion between the several modes of 

 growth is perceived. Thus, in the Rhododendron, the leaves 

 are sometimes opposite, and sometimes alternate. In the 

 Honeysuckle, they are naturally verticillate ; but the whorls are 

 broken up, and the leaves carried to a distance from one ano- 

 ther, by anything which causes an increased developement of 



