108 ELEMENTS 01? S'l'EUCl'UBAL BOTANY. 



however, they are extremely thick and succulent ; aud 

 in the case of parasites and saprophytes, such as Indian 

 Pipe and Beech-drops, they are usually either white or 

 brown, or of some colour other than green. The scaly 

 leaves of underground stems are also, of course, destitute 

 of colour. The green colour is due to the presence, of 

 granular particles of a substance called chlorophyll. It 

 is formed, as a rule, only in those parts which are 

 exposed to the action of sunlight, and it is intimately 

 connected with the process of assimilating nutritious 

 matter for the plant's use during growth. Further 

 reference will be made to it later on. 



157. As a general thing, leaves are 

 extended horizontally from the stem or 

 branch, and turn one side towards the 

 sky and the other towards the ground. 

 But some leaves are vertical, and in the 

 case of the common Iris (Figs. 88 and 89) 

 each leaf is doubled lengthwise at the 

 base,and sits astride the next one within. 

 ^'^' ^' Such leaves are called equitant. 



158. Phyllotaxis or Leaf-Arrangement. As to 



their arrangement on the stehi, leaves are alternate when 

 only one arises from each node (Fig. 3). If two are formed 

 at each node, they are sure to be on opposite sides of the 

 stem, and So are described as opposite. If, as in Mint 

 and Maple, each pair of opposite leaves stands at right 

 angles to the next pair above, then the arrangement is 

 decussate. Sometimes there are several leaves at the same 

 node,in whichcase they axewhorled or verticillate(¥i3,.\iQi). 



Fig. 140. — Whorled leaves of Galium. 



