ROOT— STEM. 



153 



(exogens) these take the place of the pith, which rarely 

 persists. The outermost cells harden into a skin, called 

 Epidermis (Gr. outer skin). New cells are constantly 

 formed at the extremity of the root; through these ab- 

 sorption is chiefly carried on, though the epidermis absorbs 

 also. The new cells are most active in spring and sum- 

 mer, when the plant is growing. In winter, when tlie 

 plant rests, they rest also ; the fibrils die with the leaves, 

 to be renewed with them again in the spring. The tips 

 of the fibrils — called Spongioles — have no epidermis. 



374. The root grows by extension at its extremity only, thus pene- 

 trating the soil sometimes to a great depth. Its growing force is tre- 

 mendous. Eocks are often split asunder by the energy of its tiny 

 cells ; it blasts and undermines with a patient sliill surpassing that of 

 the most accomplished engineer. This little sapper and miner is pro- 

 vided with an armor of thicli:, strong cells, called a Pileorhiza (Gr. 

 root-cap), which it uses not only as a weapon of defence, but as a tool 

 for excavations (Fig- 222, p). 



375. The long fibrous roots of the Blue-grass pierce through stiflF 

 clay to a depth of several feet, making this the most valuable of mea- 

 dow-grasses, on account not only of its sweetness, but of its perennial 

 habit. The winged seed of the Mahogany, falling among rocks, 

 sprouts in some fissure and grows to such size and with 

 such strength that the stone is rent as if by gunpowder. 

 Thus anchored, and feeding almost literally upon air^ 

 the tiny plant in a few years becomes the magnificent 

 and valuable tree. 



376. The Stem. — Acrogenous stems, as we know, are 

 chiefly cellular ; they have a few wood-bundles in a 

 broken or sometimes in a continuous circle near the 

 circumference (Fig. 42) ; in these woody tissues occur 

 the scalariform ducts (Fig. 220, C). They are of little 

 economic use to man, except in their fossil form of coal ; 

 but as teachers they are of the highest value, showing 

 the gradual development in the scale of organic life. 

 Gym/nogenous stem,s are partially exogenous (81). 



377. Endogenous stems (Fig. 61) are composed of cel- 

 lular tissue interspersed with wood-bundles. The growth 

 here, as in all other plants, is at first entirely cellular ; 

 wood-fibres, however, are soon developed (Fig. 223) ; 

 they are formed in the leaf, and carried down, first in- 

 ward and then outward across the older wood-bundles, pio. 22 3.— 

 which are thus pushed to the circumference, where they Stem of Iris 

 make a hard and usually inseparable rind. This rind ^^Sy! ""' 

 serves as bark, though it is really wood of the most 



durable quality, as in the palm, bamboo, and cane. The central part 

 sometimes remains cellular, forming a cylinder ; but it is not strictly 

 pith, because it has neither medullary sheath nor rays. As the stem 



