538 LIFE-HISTORIES 
Fiac. 365.—Fern Embryo (Pteris sp.). A, embryo removed from archegon- 
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 (f) 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 (f) 
embedded in the archegonium (aw), the root (w) with its tip protected 
by a root-cap, the stem (s) and the incurved leaf (b). 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 prosenchyma is par- 
ticularly well-developed and shows plainly a differentiation of 
tissues which is characteristic of all plants higher than bryophytes. 
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 fibrovascular! 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 mainly for 
conducting fluids, ferns and higher plants often develop strands or 
layers of hardened, thick-walled cells whose function is mainly to 
give strength or afford protection. Such tissue is termed scleren- 
chyma? in general, or sclerotic parenchyma or prosenchyma in 
particular. An outer layer of the cortex as at (FZ) often becomes 
sclerotic and thus contributes much additional strength to a co- 
lumnar organ. The parenchyma of a fern-stem serves very largely 
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 
1 Fi’/bro-vas’cu-lar <_ L. fibra, a fiber; vasculum, a small vessel. 
* Scler-en’chy-ma < Gr. skleros, hard. 
