58 BOTANY 



arities of their flattened form. We have seen wherein 

 lie its advantages, but we must consider also the diffi- 

 culties and even dangers which it involves. Difficulties 

 arise from the certainty that the leaves must encounter 

 rough weather in the course of the year. Rain may 

 soak them through, wind may tear them apart, or even 

 strip them from the twigs. How are these perils met ? 



FIG. 21. Diagram of forms of cymose branching. 



There are two reasons why rain falling upon them 

 does not affect them seriously for a long time. Gener- 

 ally the shape of each is such that there is a longitudinal 

 groove all along its upper surface, running from apex to 

 base in the centre of the flattened blades. This con- 

 ducts the water away as fast as it falls upon the leaf, 

 either towards the apex or towards the base. In the 

 latter case the groove is continued along the leaf stalk 

 so that the water is taken to the ground. The second 

 reason is that the outer layers of the walls of the cells 



