. Plants and their uses; an introduction to botany. Botany; Botany, Economic. 538 LIFE-HISTORIES. Fiu. 365.—Fern Embryo (Plcris si>.). 4, crnljryo renicn-i'd fruni archcgon- ium and cut vertically to show the first dividing wall (I, I) and the walls at right angles to this (II, II) whereby the fertilized egg-cell was divided into quadrants of which one (/) by further cell-division and growth becomes the foot, another (s) the stem, another (fc) the first leaf, and another (w) the root. B, embryo still further developed but still attached to the prothallus (pr). cut vertically to show the foo
. Plants and their uses; an introduction to botany. Botany; Botany, Economic. 538 LIFE-HISTORIES. Fiu. 365.—Fern Embryo (Plcris si>.). 4, crnljryo renicn-i'd fruni archcgon- ium and cut vertically to show the first dividing wall (I, I) and the walls at right angles to this (II, II) whereby the fertilized egg-cell was divided into quadrants of which one (/) by further cell-division and growth becomes the foot, another (s) the stem, another (fc) the first leaf, and another (w) the root. B, embryo still further developed but still attached to the prothallus (pr). cut vertically to show the foot (/) embedded in the archegonium (aw), the root (w) with its tip protected by a root-cap, the stem (s) and the incurved leaf (6). Magnified. (Hofmeister.) leaves and roots. When a leaf falls off it leaves a scar upon which one may see clearly traces of these slender branches which went into the petiole. In the trunk of a tree-fern (Fig. 367) the prosenchynia is par- ticularlj"- well-developed and shows plainly a differentiation of tissues which is characteristic of all plants higher than brj'ophytes. Each strand is here found to contain tliick-walled woody fibers (FB) and larger cells (VS) called vessels which have thin walls variously strengthened by ridges. These vessels correspond to the "pores" found in the wood of oak and other trees we have already studied. Such strands are called fihrovascidar'^ bundles, and the plants cr parts containing them are said to bo vascular. The ultimate branches of the framework of a leaf are often nothing but single vessels. Be- sides the woody and the vascular tissues, which serve mainlj' for conducting fluids, ferns and higher plants often develop strands or layers of hardened, thick-walled cells v.'hose function is mainly lo give strenp^th or afford protection. Sucli tissue is termed sclcrcit- chyma ' in general, or sclerotic parenchyma or prosenchyma in particular. An outer layer of the cortex as at {FL) often becomes sc
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