ORGANIC EVOLUTION 147 



and consolidated by their over-lapping pointed ends and 

 form the longitudinal wood fibres. The cells lying between 

 the bundles become transversely elongated, and form the 

 wood fibres of the medullary rays. Thus, there is formed 

 beneath the cambium a ring of wood, that owes its solidity 

 to the close adhesion of cells having lignified walls, and 

 the beauty of its grain to the arrangement of the cell 

 groups. 



The annual rings of wood are formed by the development 

 of vessels of larger diameter in the "spring wood" formed 

 during the early part of the growing season. As new layers 

 of wood are added outside, those first formed become 

 changed from "sap wood" into "heart wood," losing their 

 capacity for conducting water. That the heart wood is not 

 essential to the life of the tree, every hollow tree testifies. 

 That the outer layers are essential is shown by the fatality 

 of "girdling" the trunks by cutting a groove through the 

 sap wood and bark. 



Monocotyledons. There is, however, one great group of 

 seed plants, known as the Monocotyledons, that has not the 

 mode of growth above described. Structurally these are 

 much more like the pteridophytes. They have no cambium 

 ring, and no axis of solid wood, but their bundles are scat- 

 tered through a soft internal parenchyma, and the chief 

 support of the stem is a stiffened cortex beneath the epi- 

 dermis, comparable to the layer similarly situated in the 

 rhizome of the fern (fig. 93). 



These are the grasses and sedges, the lilies and irises, and 

 most other plants that have parallel veined leaves. These 

 dominate considerable portions of the earth's surface, in 

 prairies, steppes, savannas, marshes, and other unforested 

 regions. In temperate climes the aerial stems of all of 

 them are of annual growth, and the perenniel roots and 



