S8 PLANT LIFE. 



a waterproof epidermis which prevents the absorption of 

 water; consequently some means of supplying the above- 

 ground portion witli the required amount of water became 

 necessary. This was effected by a further division of labour 

 and differentiation of a portion of the stem tissue into fibro- 

 vascular bundles. Other things being equal, the advantage 

 of an erect stem to a plant growing on dry land is obvious ; 

 it removes those indispensable organs, leaves, beyond the 

 reach of herbivorous animals, and at the same time places 

 them in a favourable position with regard to light, which is 

 necessary for the performance of their functions. The weak 

 point in connection with the presence of a stem is the separ- 

 ation of the root, the only source of water to the plant, and 

 the leaves and other parts requiring the water, and this 

 difficulty was overcome, as already shown, by the conception 

 and perfection of the fibro-vascular bundles, which, in addi- 

 tion to the functions already enumerated, are concerned in 

 the diffusion of assimilated food-materials, giving strength, 

 rigidity, etc. 



Growing-points, as already defined, consist of primary 

 meristem or cells capable of dividing, and it is out of these 

 cells, originally all alike, that the most diverse tissue-systems 

 are formed. The majority of leaves, for example, consist 

 at first entirely of meristem ; during their development the 

 whole of this becomes differentiated into the various tissue- 

 systems constituting a fully developed leaf, so that none of 

 the generating tissue or meristem remains ; hence leaves do 

 not exhibit periodic growth. The same applies to fruits and 

 many other organs. On the other hand, in organs that con- 

 tinue to increase in length for a considerable period, as most 

 stems and roots, new meristem is being constantly formed 

 at the tip or growing-point, which furnishes the tissue that 



