THE DIFFEKENTIATION OF THE PLANT-BODY 23 



modifications of structure in such plants ; the epidermis is 

 hardly at all cuticularised, so that water can pass into its 

 cells ; while the vascular bundles are comparatively feebly 

 developed, the woody part of them being particularly 

 small. 



A third requirement of a plant of considerable mass, 

 especially if it has a terrestrial habitat, is a power of resist- 

 ing such external forces as 

 would lead to its uprooting, 

 which must be combined with 

 a considerable degree of flexi- 

 bility, at any rate at the ex- 

 tremities of the body. This 



FIG. 27. DIAGRAM OF THE COURSE 

 OF THE VASCULAR BUNDLES IN 

 AN HERBACEOUS DICOTYLEDO- 

 NOUS PLANT. 



FIG. 28. DISTRIBUTION OF THE 

 VASCULAR BUNDLES OR VEINS 

 IN A FOLIAGE LEAF. 



combination of rigidity and flexibility has been secured in 

 various ways, varieties of both the form and the structure 

 of the plant being concerned in it. In the simplest plants 

 but little differentiation of the body is needed ; such forms 

 as consist of single cells, or rows or plates of cells, living in 

 water, need hardly any rigidity, and in their cases the 

 unthickened cell-wall yields sufficient support to the proto- 

 plasm. Larger plants which grow in rapidly flowing water 

 usually possess flexible stems and much-divided leaves, which 

 consequently give way to the current, and escape damage. 

 Small terrestrial plants or parts of plants, which have but 



