230 ELEMENTARY STUDIES IN BOTANY 



verticillate) . In the case of cyclic leaves, the cycle of one 

 node does not stand directly over the cycle of the node just 

 below it, but over the spaces between the leaves below. All 

 these terms, however, need not confuse, for the fundamental 

 fact is that leaves appear at the nodes in spiral succession, 

 the so-called " spiral " or " alternate " leaves referring to a 

 single spiral of leaf succession ascending the stem from node 

 to node, and the so-called " cyclic," " opposite," " whorled," 

 or " verticillate " leaves referring to two or more spirals of 

 leaf succession ascending the stem from node to node. 



FIG. 198. Prostrate stem of Potentilla. 



131. Stem-position. It is obvious that the ideal position 

 of a stem for leaf display is a free erect position, for leaves 

 can be displayed freely on all sides. Of course to maintain a 

 free erect position is not a simple mechanical problem, and in 

 such stems this problem must be added to the universal one 

 of water-conduction. That erect aerial stems must be con- 

 structed to maintain their position becomes evident when 

 they are contrasted with erect submerged stems. In small 

 lakes and slow-moving streams such submerged stems are 

 commonly seen, as the pickerel-weed and numerous other 

 forms. In the water their stems are erect, but when taken 

 out of water they collapse, having been maintained in posi- 

 tion by the buoyant power of water. 



