CONDUCTION. 262 CONDUCTOR. stem may be cut without such a result. A solu- tion of indigo-carmine or eosin stains only the xylem strands when the cut end of a leafy shoot is plunged into it. It ha>i further been sho«Ti that the water travels chiefly in the cavities of the trachete, though doubtless a portion traverses the walls themselves, and all must pass through many partition walls in its course. The move- ment is not at all like that of water in pipes or blood in vessels. The force concerned in the movement of water is not certainly knomi. In the smaller plants, root-pressure (q.v. ) may co- operate, or under some circumstances may be sufficient; but it is not adequate to account for the rise in tall trees. Capillarity is to be ex- cluded, since the conditions luider which surface tension operates to raise water in small tubes are not present. Evaporation and the suction set up thereby play an important part; and jirobably the most important factor is the osmotic action in the living cells of the leaf. But no sat- isfactory explanation of the observed facts has as yet been found. The Phloem and Food-Conduction. The es- sential elements of the phloem are sieve-cells and companion-cells (parencliyma ) ; often bast fibres accompany them. Sieve-cells, like trachese, lose their living contents as they mature. The end walls, and often certain areas of the lateral walls, where they abut upon a like cell, become resorbed in spots and perforate. The perforate area is known as a sieve-plate. Through the perforations the contents may pass freely. It has been shown that when the perforations in a membrane amount to less than 1 per cent, of the area, there may pass throvigh it 60 per cent, of the gases and solutes which could do so were the whole area open, provided they are being freely removed from the other side. In life the con- tents of the sieve is a slime, in which are abtm- dant the various foods, both proteid and carlio- hydrate. In addition to this evidence as to their function, girdling experiments have sho^vn that when the phloem bundles are severed the growth of parts beyond is hindered or stopped. Other tissues of the bundles participate in the transfer, but no decisive division of labor can be made out, beyond the fact that the sieve-cells are most efficient. Latex-Vessels. The latex-vessels are irreg- ularly branched tubes (the branches sometimes connected into a network), containing a milky or colored sa]) called latex (q.v.). They are found in only ten of the large families of plants, both monocotyledons and dicotyledons, in which They are believed to ser'e for the conduction of foods. The reasons for this are as follows: (1) The latex always contains, among a great variety of substances, foods whose amount varies much from time to time, and these variations seem to be correlated with variations in growth and de- velopment. (2) The latex- vessels are developed early among the growing tissues where foods are needed. They have abundant branches in the leaves and special relations to the cells where food is made. The mode by which the foods are transported in sieve-cells and latex-vessels is not known. The contents are under pressure from the tirgor (q.v.) of adjacent living cells. Bending and the consequent mechanical compi'ossion would facili- tate mixing of the contents. But diffusion move- ments are probably the main agency. CONDUCTION OF HEAT. See Heat. CONDUCTOR (Lat., leader, from comlucere, to lead together). In nnisic, the person who di- rects the chorus or orcliestra, or both combined, and wlio is responsible for the interpretation of the works performed by the artists untler his direction. A' good conductor must be a thorough musician. He nuist have had careful training in all branches of musical composition, must be familiar with the compass and peculiarities of the voice and all orcliestral instnnnents ; must be a good score-reader, and a man of broad nnisical culture, familiar with the styles of various epochs and masters. In addition, he must be gifted with poetic temperament, an uiuisually tine car, a forceful, magnetic personalitj' that com- mands instant obedience, and great coolness and presence of mind. That he be also a tine per- former on some instniment is not essential ; two of ,the world's greatest conductors, Wagner and Berlioz, were wretched performers. The prin- cipal work of the conductor is not done in public during performances, but during rehearsals. His preparation really begins at home. He must make himself thoroughly familiar with the score of the work he is to conduct. This is best done at the pianoforte. He must have a clear idea of the form of the work, of the melos (q.v.), of the different phrases. Before he conducts the first rehearsal, he has decided on the interpretation of the work, and knows exactly what he wishes each IJerformer to do. The first rehearsal of a new work (especially if ]ierformed from manuscript) is largely taken up with correcting mistakes in the parts. Here the conductor's ear must be on the alert. During rehearsal the conductor can convey his instruc- tions to the singers and instrumentalists by means of the spoken word, audible beating of the rhythm, and by singing or playing to them. In choral works the chorus is rehearsed separately with the piano. The soloists also rehearse pri- vately, with the conductor at the piano, before rehearsals with full orchestra begin. In study- ing instrumental works, like symphonies, a care- ful conductor often rehearses the strings and wind instruments separately. After the perform- ers have become thoroughly familiar with the con- ductor's intentions, they are ready to be guided during the public performance by his baton, and by signals given with the hand or eyes. By that time the conductor practically knows the score by heart. It lies before him more for occa- sional reference than actual reading. People in general know very little about the real respon- sibility and importance of the conductor. Berlioz does not exaggerate when he says that a poor singer or instrumentalist can ruin only his or her part, but a poor conductor can ruin the whole performance. While the essential functions of the conductor have been pretty much the same at all times, the manner of conducting has varied greatly. The custom of beating time with a baton can be traced to the remotest antiquity, when oarsmen were directed by such means. When the baton was introduced for beating time in music is not kno^ftm. An ancient manuscript is preserved in Paris, showing Heinrich von Jleissen, a minne- singer, who died in 1318, directing a group of vocal and instrumental performers by means of a baton. We know nothing of the mode of con- ducting between that time and 1600. The earliest
