THE DIFFERENTIATION OF THE PLANT-BODY. 23 
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 
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Fie. 27.—D1aGRAM OF THE COURSE 
OF THE VASCULAR BUNDLES IN Fic. 28.—DisTRiBUTION* or THE 
AN Herpactous DicoTyLEDO- VascuLar BUNDLES OR VENIS 
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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 affords 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 
a short life, resemble these aquatic forms in their general 
characteristics, though they show much greater variety in 
the forms of their leaves. The rigidity and flexibility of 
