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Show G INTRODUO'l'ION. lower surface, and thus causes it to bend downwards. Ilyr,onusly is the reverse, and implies increased growth along the lower surface, causing the part to bend upwards.* . 1l1ethods of Observation.-Tbe movements, so~ctimes :cry small and sometimes considerable in extent, of tho. varwus organs observed by us, were traced in t_he manner wh1ch ~ftcr many trials wo found to be best, and which must be dcscr~bed. Plants growing in pots were protected wholly _from tho hght, or had li()'ht admitted from above, or on one side as tho case might re~uire, and were covered above by a large ho~·izontal sheet of glass, and with another vertical ~heet or: one sHle. A glass filament, not thicker than a horsehair, and from a quarter to three-quarters of an inch in length, was affixed to the pn.rt to be observed by means of shellac dissolved in alcohol. The solution was allowed to evaporate, until it became so thick that it sot hard in two or three seconds, und it never injmecl the tissues, oven the tips of tender radiclos, to which it was applied. To the end of the glass filament an excessively minute bead of black scaling-wax was cemented, below or behind which a lJit of card with a black dot was fixed to a stick driven into tho ground. Tho weight of the filament was so slight that even smal~ leaves were not perceptibly pressed down. Another method of observation, when much magnification of the movement w11s not required, will presently be described. The bead and tbc dot on the card were viewed through the horizontal or vertical glaHs-plato (according to the position of the object), and when ouo exactly covered the other, a dot was made on tho glass-plato with a sharply pointed stick dipped in thick Indian-ink. Other dots wore made at short intervals of time 11nd these were afterwards joined by straight lines. The figures thus traced were therefore angular; but if dots had been made every 1 or 2 minutes, the lines would have been more curvilinear, as occuncd when radicles were allowed to trace their own courses on smoked glass-plates. To m11ke the dots accurately was the sole difficulty, and required some practice. Nor could this be done quite accurately, when the movement was much magnified, such as 30 times and upwards; yet even in this case the general course may be trusted. To test tho accuracy of the above method of observation, a filament was fixed to an * 'rhese terms are used in the sense given t!Jem by De Vries, ' Wiirzburg Aruoilcu,' 11oft ii. 1872, p. 252. INTRODUC'fiON. 7 inanimate object which was made to slide along 11 straight edge and clots wore repeatedly made on a glass-plato; when these were joined, the result ought to h11vo been a perfectly straight line, and the line was very nearly stmight. It may be added that when the dot on the card was placed half-an-inch below or behind tho be11cl of scaling-wax, and when the glasspl11te (supposing it to have been properly curved) stood at a distance of 7 inches in front (a common distance), then the tmcing represented the movement of the bead magnified 15 times. Whenever a great increase of the movement was not required, another, and in some respects better, method of ob ervation wa~ followed. This consisted in fixing two minute triangles of thin paper, about 2 1 0 inch in height, to the two ends of the attached glass fil11mcnt; and when their tips wore brought into a line so that they covered ono another, dots were made 11s before on the glass-plato. If we suppose the glass-plato to stand 11t a dist~: tnce of seven inches from the end of the shoot bearing the filament, the dots when joined, will give nearly tho same figure as if a filament seven inches long, dipped in ink, had been fixed to the moving shoot, 11nd had inscribed its own course on the plate. The movement is thus considerably maO'nificd · for instance, if a shoot one inch in length were bcndi~g, and the glass-plate stood at the distance of seven inches, tho movement woul~ be magnified eight times. It would, however, have been very difficult to have ascertained in each c11se how great 11 length o~ ~he shoot was bending; and this is indispensable for ascertammg the degree to which the movement is magnified. After dots had been made on the glass-plates by either of ~~e above mcth_ods, they wore copied on tracing paper and JOmed by ruled hnos, with arrows showing the direction of the movement. Tho nocturnal courses are represented by straight broken lines. The first clot is always made larger th11n the others, so as to c11tch the eye, 11s may be seen in the diagrams. The figures on the gl11ss-plates were often drawn on too large a scale to be reproduced on the pages of this volume 11ncl the proportion in which they have been reduced is u.lwa;s given.* Whenever it could be approximately told how much tho movement had been magnified, this is st11ted. We have perhaps * We arc much indebted to Mr. Cooper fur the care with which be has reduced and engraved our diagrams. |