138 



PHYSIOLOGICAL BOTANY. 



The cauttex of Endogens often takes peculiar forms. The rind is capable of only a 

 limited expansion. This limit is soonest attained at the base of the stem, long before 

 the upper parts cease to enlarge. Consequently such trunks are often seen of equal or 

 j;reater diameter at the summit than at the base : so the Palmetto, Corn, Bamboo. 



425. The acrogenous structure is found in Mosses, Ferns, 

 and the other higher tribes of the Cryptogamia. The stems ad- 

 vance, beneath or above the ground, full-formed, growing only 

 at the end, hence called Acrogem. 



A cross-section of a Fern stem shows a body of parenchyma strengthened by an outer 

 zone of fibro-vascular bundles, the whole invested with a sort of bark. The bundles are 

 precisely similar to those found in the petioles, showing that the stem is the aggregate of 

 the unaltered leaf-bases (514). 



426. Tliallogens are the lowest in the scale of rank, having no true axis and no 

 other tissue than parenchyma, which grows in threads or in mass in all directions. The 

 apparent stems (the stipes), if there be any, support the fructification only. Such are 

 Sea-weeds, Lichens, Mushrooms, Puff-balls, Frog spittle, and Mildew. 



427. The structure of roots presents few deviations from that of the stems to 

 which they severally belong, being exogenous in Exogeus, endogenous in Endogcns, etc. 

 In the former class the central pith disap- 



pears, its place being occupied mainly by 

 vascular ducts ; and the liber, if any, has no 

 bast-cells. 



428. The FIBRILL^E and pileorhiza should, 

 however, be mentioned as peculiar in the 

 structure of the root. The former are pro- 

 duced by millions, clothing the delicate epi- 

 dermis of the young rootlets as with a cottony 

 down, especially in light soils. They usually 

 consist of a single cell of the epidermis ex- 

 tended, as seen in figure 515. They are the 

 true absorbents, the mouths of the growing 

 plant. 



429. The microscope shows that the ex- 

 treme, advancing point of the delicate, grow- 

 ing fibres is not thrust naked against the 

 opposing soil, but ia covered with a cap 

 called PILEORHIZA (pileus, a cap, rhiza, root), 

 which consists of older, hardened cells, be- 

 hind which are formed the new cells. In 

 the Duckmeat, the pileorhiza is lengthened 

 into a sheath. 



430. The manner of growth in the root is not like that of stems, by the extension of 

 parts already formed, but simply by the addition of new matter at the advancing point. 

 This accounts for the wonderful facility with which it penetrates the soil and finds ita 

 way uninjured into the hardest earth. 



431. Structure of Leaves. The leaf may be regarded as 

 an expansion of the two outer integuments of the bark, or of the 

 green bark and the epidermis, expanded into a broad, thin sur- 

 face by a woody framework proceeding from the medullary 

 sheath and the liber. The framework of veins is therefore fibre 



515, Extremity of the rootlet of Maple, with 

 fibrillse and (.) pileorhiza. 516, Two plants of 

 Letnna minor (Duckmeat) , their pileorhiza. 



