268 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- 
