538 



LIFE-HISTORIES 



Fig. 365. — Fern Embryo (Pteris -sp.). A, embryo removed from archcgon- 

 ium and cut verticallj- 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 (b) 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 (»•) 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 clearh' traces of these slender branches which went 

 into the petiole. 



In the trunk of a tree-fern (Fig. 367) the prosenchyma is par- 

 ticularly well-developed and shows plainly a differentiation of 

 tissues which is characteristic of all plants higher than br3'ophytes. 

 Each strand is here found to contain thick-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 fibrovascidar ' bundles, and the plants or 

 parts containing them are said to be 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 mainh' for 

 conducting fluids, ferns and higher plants often develop strands or 

 layers of hardened, thick-walled cells whose function is mainl}' to 

 give strength or afford protection. Such tissue is termed scleren- 

 chijma - in general, or sclerotic parenchyma or prosenchj'ma in 

 particular. An outer layer of the cortex as at {FL) often becomes 

 sclerotic and thus contributes much additional strength to a co- 

 hunnar organ. The parenchyma of a fern-stem serves very largeh' 

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

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

 growths which serve mainly to protect organs that are very young 

 or especially need to be covered. Whereas in multicellular plants 



' Fi'bro-vasVu-lar < L.fibra, a fiber; rasculum, a small vessel. 

 - Scler-en'chy-ma < Gr. skleros, hard. 



