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 

 sclerotic and thus contributes much additional strength to a co- 

 lumnar organ. The parenchyma of a fern-stem serves very largelj' 

 for the storage of reserve food in the fornt of starch. From the 

 epidermis of various jiarts may arise hair-like or scale-like out- 

 growths which serve mainly to iirotecl organs that are vny >'oung 

 or especially need to be covered. Wlicrcas in nuilticellular plants 



' Fi'l)ro-vas'fu-l,-Lr < \j. filyrn, a liber; rdxciihiiti, a small vessel. 

 - Scler-en'chy-ma. < Cir. sldcratt, hard. 



