THE LIGNEO0S SYaiBM. 



139 



637, Eootlet of Madder, showing cells expanded into flbrilliE. 58S, Grlandular Iiair of Fraxi- 

 nella, section. 589, Hair of Bryonia, of several cells. 590, Hair of several cells, surmounted by 

 a gland, of Antirrhinum majus. 591, Sting of Urtica dioioa. 692, Jointed hair of the stamens of 

 Tradescantia. 59.S, Stellate hair from the petiole of Nuphar advena (magnified 300 diameters, 

 Henftey). 694, Branched hciir, one cell, of Arabis. 



OHAPTBK IV. 



THE LIGNEOUS SYSTEM 



685. Includes the firm structures of roots, stems, and their append- 

 ages, summarily called the wood. 



686. Steuotokb. The growing rootlet of the germinating plant exhibits under 

 a microscope a nearly uniform mass of cellular tissue. The cells composing it are 

 soft and delicate, with thin, porous walls adapted to absorb moisture, which it has 

 already begun to do. It grows by the accession of cell to cell through their divi- 

 sion and enlargement at its point, or rather just behind the advance layer which 

 constitutes its cap (pileorhiza § 725. 



68Y. TnE EARLIEST TISSUE. The same structure also appears in the expanding 

 cotyledons and the opening bud of the plumule. At this early stage, therefore, all 

 plants alike in all their parts are composed of simple parenchyma. Subsequent 

 changes in structure occur, giving to each tribe its several peculiarities. Still the 

 growing points of the axis, both ascending and descending, advance by the format 

 tion of the same tissue, and the vessels, if formed at all, follow a little later. 



688. The changes. The rootlet soon becomes a root, assumes a corky layer in- 

 stead of the tender, spongiform epidermis, and ceases to absorb; But new rootlets 

 spring from the radicle, or branch from the axis, which in their turn absorb, harden, 

 divide and subdivide ; and so on indefinitely. 



689. The isceeasing demand for moistube is thus met by the multiplica- 

 tion of these root ends, which have been called the spongehts. The absorbing sur- 

 face is also greatly increased by the hair-Uke processes of the epidermis ; — ^the 

 fibrillae (§ 724) which multiply generally in proportion to the dryness of the soiL 



690. There are four general modes of growth and structure, 

 whereby the vegetable kingdom is distinguished into as many great 

 classes, viz.: 



