so MORPHOLOGY 



of tropical forests: they seem to make as it were 

 frantic efforts to reach to the tops of the trees on 

 which they climb for the purpose of exposing their 

 leaf-surfaces freely. The same effort is seen in trees 

 that grow in thick forests. A Mango tree, for ex- 

 ample, growing singly and another growing in a 

 Mango tope well illustrate the fact. Common experi- 

 ence also shows that ordinarily no leafy plants grow 

 in the shade, or, even if they do grow, they soon 

 become sickly and pale and ultimately die. 



Leaves that are inserted spirally on the stem show 

 a remarkable method in their arrangement. In 

 Grasses, for instance, the leaves are placed on the 

 stem in two vertical lines or orthostichies. The 

 distance between the insertion of two successive leaves 

 measured round the stem is one-half the circumfer- 

 ence of a circle. This distance is termed lateral 

 DIVERGENCE, and when there are two orthostichies 

 is expressed by the fraction |; and such an arrange- 

 ment is described as distichous. , Amlaki {Phyllan- 

 thus) and dulal-champa {Hedychium coronariuvi) arie 

 other examples of it. Similarly, the arrangement is 

 TRiSTiCHOUS when the leaves are arranged in three 

 orthostichies and the lateral divergence is \ ; penta- 

 STiCHOUS, with leaves arranged in five orthostichies 

 and the lateral divergence f; and so on, higher 

 and higher. Thus it is | in Papaw, y\ in amrha 

 {Spondias mangifera), and so on. In all these cases 

 the distance measured spirally round the stem, 

 through the insertion of the successive leaves, from 

 any given leaf as a starting-point to the leaf which 

 is immediately above it in the same orthostichy forms 

 what is known as a cycle, the second leaf in the 

 same orthostichy forming the starting-point of the 

 next cycle and being included in the latter. In 



