48 



♦ KNOVSTLEDGE ♦ 



[Dec. 1, 1885. 



PLEASANT HOURS WITH A MICROSCOPE. 



Br Henry J. Slack, F.G.S., F.R.M.S. 



T this season of pear-e.iting every one 

 becomes acquainted with the fact that 

 this delightful fruit contains, besides the 

 luscious, succulent matter, some gritty- 

 stuff, especially near the core, but only 

 these who have examined it under a 

 microscope have any idea of its interest 

 and beauty. The bulk of a good pear consists of thin 

 cells filled with the fruity material which is readily 

 digested ; the gritty matter called sclerot/tn, or hard 

 tissue, is as indigestible r,s bits of tough wood, but useful 

 as a mechanical stimulant of the intestine. In the for- 

 mation of a plant-cell the first process is the deposition 

 of particles to make a thin, delicate tissue. This, when 

 required, is thickened, not by simple additions of layer 

 upon layer, as successive sheets of paper might be 

 stuck together to make a strong body, but by the newly- 

 formed tissue taking in fresh molecules. In many cases 

 all that a plant requires for some of its cells is that they 

 shall be able to hold living soft matter, and carry on 

 processes of endosmose and exosmose — in and out pas- 

 sage of fluid matter — through their delicate walls. 

 Minute differences in the arrangement of the cell mole- 

 cules, which utterly batHe microscopic investigation, 

 invest cell walls with varying properties, which enables 

 them to give a preference to the entrance or exit of 

 different materials, and to act with different degrees of 

 velocity. 



Besides cells containing living and active matter, 

 plants require to be strengthened, so as to give firm- 

 ness and support ; others to unite and construct vessels 

 and pipes of different sorts ; some kept open hx spiral 

 fibres, like the wire inside flexible gas-pipes ; others to 

 be hardened by special deposits. 



The growth of these various cells occurs through the 

 activities of matter containing a large proportion of 

 water. Every structure is built up with particles ren- 

 dered mobile by fluids, and, as many minerals cannot 

 crystallise without taking up a definite quantity of water, 

 called water of crystallisation, so plants in forming organs 

 need what has been called water of organisation, in this 

 respect resembling animals. When a hard tissue is 

 formed, none of the strong matter is deposited without 

 the help or agency of soft matter. The process is not 

 like sticking a layer of wood on to a soft one of cotton, 

 but one in which soft active matter becomes penetrated 

 by the firmer.particles. Plant-growth takes place in two 

 or more definite lines of direction, and not in one only. 

 Here we find a process like that of mineral crystallisa- 

 tion, in which molecules are built up in patterns deter- 

 mined by the directions in which the cohesive and repul- 

 sive forces opei-ate. 



If we cut a little piece out of a pear, near the core, we 

 are sure to find a considerable number of the sclerogen 

 cells, aggr.^gated in little masses, some the size of a small 

 pin's head, others much less. The biggest are formed 

 by the coalescence of a considerable number of 

 the smaller ones, and look much like imperfectly- 

 cry.stallised milk-suga;-. The smallest are composed 

 of a multitude of cells, very hard, traiispiirent, like 

 glass, and exhibiting, when magnified, branehiug canals. 

 Each hard cell is surrounded by a fringe of thin, delicate 

 cells, seen to ba pitted when magnified two or three 

 hundred times, Tie annexed figures, borrowed from 

 Quekett, show the character of the two kinds, s,nd the 

 ob eryoi' slioiild coniP-i'o the gclerogen ctHs with tjios-- 



of the bone of any mammal, which they somewhat 

 resemble. 



A small piece of pear full of these sclerogen cells 

 should be boiled in water for some time. When the 

 pulp is very soft, the sclerogen masses are easily picked 

 out with needles, and should be well-washed to remove 

 the succulent matter. Examined in this state under a 

 power of about fifty linear, they might be taken for 

 aggregations of imperfect crystals. They should then be 

 crushed in a small pestle and mortar if one is at hand ; if 

 not, wr.ip]5ed in paper and pressed hard with a knife- 

 handle. If an attempt is made to crush them w ith a 

 knife-blade they will start off, as both they and the knife 

 are elastic. The crushing should be done without much 

 grinding, or the specimens will fce spoilt. When broken 

 up, the particles should be examined with a hand-magni- 

 fier, or a dissecting microscope, and a dozen or so picked 

 out which have been separated from the mass without 

 injury. These can be mounted in Canada balsam, and are 

 well worth preserving. By transparent illumination the 

 fine canals look dark, by dark-ground ditto they come out 

 light and glittering. 



The function of the sclerogen masses seems to be that 

 of strengthening the surroundings of these ovaries, and 

 protecting the seeds. A similar growth of sclerogen 

 gives hardness to the shells of stone fruit. 



The student should make sections of peach-stones and 

 similar things, and examine those of the ivor3--nut ; the 

 fruit oi phi/ttiephas inacrocarpa, a pretty palm-like plant, 

 the ivory tree of S. America. The compound teeth of some 

 animals, such as the Cape &Vii-ei\Xev (_Or)icteropus captiisis), 

 and of the rays afford instructive comparisons. The 

 ivory tree (Pli/tetepihus) is hardened albumen. It is soft 

 when the nut is fresh-gathered. The meat of the cocoa- 

 nut is a similar structure with the hardening not carried 

 bej'ond the tough state. Thin sections of pear-.seeds 

 should be examined. Their albumen is comparatively 

 soft, and its cells very different from those of the 

 sclerogen. 



OPTICAL RECREATIONS. 



Bi' A Fellow of the Royal Astronomical Society. 

 COLOUR AND LIGHT. 



E have seen how what we call white light 

 is i-eally composed of a mixture of co- 

 loured lights ; such colours having their 

 origin in the length of the »thoreal 

 waves which produce them; and in Fig. 2 

 (p. 16) we have represented the chief or 

 most salient colours of the spectrum as 

 consisting uf violet, blue, green, yellow, and red. These 

 were long considered as primitive or undecomposable 

 ooloars. By-and-by, though, wo shall see that (to adopt 

 a phrase from iirithmetic) wo have not even yet reduced 



