VEN 



VEN 



velocity of a body is called absolute, if we 

 consider its motion in space, without any 

 reference to that of other bodies. It is 

 termed relative, when compared with 

 that of another body which is itself in 

 motion. If two carriages go along the 

 same road in the same direction, their 

 relative velocities will be the difference 

 of their absolute velocities; if in oppo- 

 site directions, the same. See Motion. 



2. Velocities, parallelogram of. A pro- 

 position very nearly analogous to the 

 'parallelogram of forces,' employed for 

 finding the resultant of two velocities. 

 Thus, " if two velocities, with which a 

 particle is simultaneously animated, be 

 represented in direction and magnitude 

 by two straight lines drawn from the 

 particle, the resultant velocity of the 

 particle will be represented in direction 

 and magnitude by the diagonal of the 

 parallelogram described upon those two 

 straight lines." 



3. Velocity, resolution of. It follows 

 from the proposition just stated, that, if 

 a particle be moving with a velocity v in 

 a direction making an angle 6 with a 

 given line, we may conceive the particle 

 to be animated by two velocities, v cos 

 in the direction parallel to the given line, 

 and V sin 9 in the direction perpendicular 

 to it. This is called resolving a velocity. 



VELUM. Literally, a veil; hence 

 applied to a horizontal membrane con- 

 necting the margin of the pileus of a 

 fungus with the stipes. When it is 

 adnate with the surface of the pileus, it 

 is a velum universale; when it extends 

 only from the margin of the pileus to the 

 stipes, it is a velum partiale. 



VENATION OF LEAVES. A term 

 denoting the manner in which the veins 

 are distributed among the cellular tissue 

 of the leaf. Writers differ much on this 

 point of nomenclature : the following 

 arrangement is taken from Dr. Lindley's 

 " Introduction to Botany." Leaves are 

 called, — 



1. Veinless, when no veins at all are 

 formed, except a slight approach to a 

 midrib, as in mosses, fuci, &c., and the 

 lowest tribes of foliaceous plants. Under 

 this head, De CandoUe has his folia nul- 

 linervia, in which there is not even a 

 trace of a midrib, as in ulva ; and folia 

 falsinervia, in which a trace of a midrib 

 is perceptible. 



2. Equal-veined, when the midrib is 

 perfectly formed, and the veins are all of 

 equal size, as in ferns. These are inter- 

 mediate between those without veins and 



362 



those in which "primary " veins are first 

 apparent. The veins are equal in power 

 to the "proper veinlets" of leaves of a 

 higher class. See Veins of plants. 



3. Straight-veined, when the veins are 

 entirely primary, generally very much 

 attenuated, and arising from towards the 

 base of the midrib, with which they lie 

 nearly parallel ; they are connected by 

 "proper veinlets;" but there are no 

 "common veinlets." The leaves of 

 grasses and of palms and orchidaceous 

 plants are of this nature. 



4. Curve-veined, a modification of the 

 last form, in which the primary veins are 

 also parallel, simple, and connected by 

 unbranched "proper veinlets;" do not 

 pass from near the base to the apex of 

 the leaf, but diverge from the midrib 

 along its whole length, and lose them- 

 selves in the margin. It is common in 

 Zingiberaceae. The straight-veined and 

 curve-veined leaves are referred by De 

 Candolle to modifications of the petiole. 



5. Netted, when all the veins of a com- 

 pletely developed leaf are present, ar- 

 ranged as described under the article 

 Veins of plants, without any peculiar 

 combination of any class of veins^ This 

 is the common form of the leaves of 

 dicotyledons, as of the lilac, the rose, &c. 

 This is the folium retinervium of De Can- 

 dolle. 



6. Ribbed, when three or more mid- 

 ribs proceed from the base, to the apex of 

 the leaf, and are connected by branching 

 primary veins of the form and magni- 

 tude of proper veinlets, as in melastoma. 

 This must not be confounded with the 

 straight-veined ]eaf, from which it may in 

 all cases of doubt be distinguished by the 

 ramified veins which connect the ribs. 

 If a ribbed leaf has three ribs springing 

 from the base, it is said to be three- 

 ribbed; if five, five-ribbed ; and so on. 

 But if the ribs do not proceed exactly 

 from the base, but from a little above it, 

 the leaf is then said to be triple-ribbed, as 

 in helianthus. 



7. Falsely ribbed, when the " curved " 

 and "external veins," both or either, in a 

 netted leaf, become confluent into a line 

 parallel with the margin, as in all myr- 

 taceous plants. This has not been before 

 distinguished. 



8. Radiating, when several ribs radiate 

 from the base of a netted leaf to its cir- 

 cumference, as in lobed leaves. To this 

 head are referred the pedalinerved, palmi- 

 nerved, and peltinerved leaves of De Can- 

 dolle ; the differences of which do not 



