Page:The New International Encyclopædia 1st ed. v. 01.djvu/603

ciilin, whioli lendors the wall partially water- proof. The extreme outer portion of the wall may be completely transformeil into eutin, con- stituting the cuticle. Besides this, the epidermal cells, when young and active, are capable of ex- creting upon the surface a lajer of waxy or resinous material, which interferes still more with the exit or entrance of water. Outgrowths from the epidermal cells in the form of hairs or scales, with which the surface is sometimes com- pletely covered, also retard evaporation. On the twigs of perennial plants the epidermis dies and withers away. Its place is then taken by sev- eral or many laj'ers of dead cells of a tabular form with waterproof walls, called cork. This is the product of a zone of secondary meristem developed vmder, or, more rarely, in the epider- mis itself. Cork gives to the stems or twigs of trees their yellowish or brownish color. On older parts it forms the outer parts of the bark (q.v.).

(2) The Absorptive System. (See Absorp- tion.) The absorptive system of the fungi con- sists mainly of the Mycelium ( q.v. ) . The Algse (q.v.) may take up materials by any part of the surface exposed to the water. In mosses and liverworts the hair-like outgrowths by which they are anchored (rhizoids) are supposed to be absorptive, but sufficient proof of this is lacking. The leaves or even the general surface of the body are the most efficient ab- sorbing regions. In the higher plants the absorp- tive system for water and its solutes is represented mainly by the root, and especially by the root- hairs. For gases, the ab- sorptive system is the whole surface of the aerial parts, but notably that of the leaves. In a few plants there are special out- growths on aerial parts in the form of hairs, which

U.mt-hairs (not yet full- »•"<= able to absorb water, grown) on the root of or even solutions of or- whwit Bcedling The pro- ganie material; but as a toplasm of cells is not , .u i. e ^ shown. The hairs adhered "'« the amount of water eo firmly to certain soil which mav be absorbed by particles that they could the aerial part of a plant is not he washed off. ,, ^ . , ^,. ., , so small as to be negligible. Very young plants ( embryos ) often have special organs for absorbing the food materials stored around tlieni in the seed. These organs, however, are transient and disappear with the exhaustion of the food supply.

(3) The Conducting System. Water and foods may be transferred from one part of the body to the other through any of its living tis- sues by dili'erences in osmotic pressure. (See Os- mosis.) In the smaller plants, these osmotic movements probably suffice, but in the large forms they are too slow, and special conducting systems have therefore been developed. These consist of strands of elongated cells extending from the neighborhood of the absorbing or food- making regions throughout the body and reaching all its parts. The conducting sj'stem is often called the fibro-vascylar system, because its es- sential elements were formerly called "vessels" (after the analogy of blood "vessels"), and these are frequently acconi])anied by mechanical ele- ments in the form of fibres. The conduction

Diagram of the transverse section of a monocotyJedonons stem (.-I.sy/rtrtfr/w*'). The ovate bodies scattered tliroiigh the section indicate the sheathed pairs of xyiem and phloem bundles.

(q.v.) of water and foods is carried on in the main by separate strands. The water-conducting

Diagrammatic longitudinal tangential section of a xylem bundle of Corn (Zea mayf), showing the surrounding parenchyma (tliin, isodianietric cells) ; the sheath (thick-walled, elongated, pointed cells) ; two pitted vessels (tracnese) ; and in the centre an annular vessel.

strands are the xylem strands, the food-conduct- ing ones the phloem strands.

The xylem strands consist of trachese, or tra- cheids, accompanied by variable amounts of parenchyma cells and often fibres. The tracheae are formed by the fusion of rows of elongated cells through the absorption of most of the abut- ting end walls. They thus become long tubes (1 to 3 meters), emptied of protoplasm at ma- turity, and with their walls irregularly thick- ened, often in elaborate patterns. The tracheids are similar, but do not suffer the absorption of the end walls, so that each is a cell and not a cell-fusion. The phloem strands consist essen- tially of sieve tubes and varying amounts of elon- gated parenchyma-cells. The sieve tubes resem- ble the tracheae in the loss of living contents, but differ from them in the more uniform thick- ness of their walls, and particularly in having only portions of the end walls (or even the side walls between adjacent sieve tubes) absorbed.