2(!8 EVOLUTION OF PLANTS 



tissue or cambium. It is interesting to note that this 

 peculiar type of growth has developed quite indepen- 

 dently in several widely separated groups of plants, 

 apparently in response to similar conditions of growth. 

 Not only do we find it in the unrelated Conifers and 

 Dicotyledons, but also in certain Pteridophytes, both 

 fossil and recent. In the Monocotyledons, when the 

 plant reaches tree-like proportions, the rigidity of 

 the trunk is brought about in part by a large develop- 

 ment of strengthening tissue in the outer part or cor- 

 tical region of the stem, and partly by the presence 

 of a great many separate vascular bundles, each of 

 which is usually surrounded by a sheath of supporting 

 cells. 



The great mass of stems and foliage in the larger 

 flowering plants necessitates a very perfect system of 

 roots, both for anchoring the plant firmly in the earth, 

 and for supplying it with water and various food 

 elements. In Gymnosperms and Dicotyledons, which 

 have woody trunks, there is very often a main or 

 tap-root which is a direct continuation of the stem, 

 and, like it, continues to increase in diameter 

 through the permanent growth by its vascular bundles. 

 In the comparatively small number of arborescent 

 Monocotyledons, like the palms and screw-pines, the 

 necessary support is given by a great many stout ad- 

 ventitious or secondary roots, which, however, are 

 usually incapable of secondary growth in thickness. 

 The screw-pines (^Pandanus) are especially remarkable 

 in the development of these roots from points far above 

 the ground, and the trunk is often supported by a great 

 number of these, which form a conical mass of buttress- 



