HARDWICKE'S SCIENCE-GOSSIP. 



159 



tricts. Mr. Gulliver stated that though the epi- 

 dermal cells of plants often afford good diagnostic 

 characters, it is remarkable that they have been 

 little used. The object of the present communication 

 was to show that these cells of Listera ovata differ 

 from those of other orchids. In this species, the 

 epidermal cells on the under surface of the leaf have 

 remarkably sinuous boundaries, so as to form a good 

 example of that common kind of epidermis which 

 botanists have named Colpenchyma, while on the 

 upper surface of the leaf of that same plant the 

 cells have smooth margins, more or less polygonal 

 from mutual pressure of roundish or oblong cells. 

 Thus, besides the stomata on the under side of the 

 leaf, the epidermis of the two sides differs so plainly 

 and curiously as to present very pretty microscopic 

 objects. At the same time, for comparison, examina- 

 tions were made of the corresponding tissue of 

 Orchis mascula. Orchis fusca, Ophnjs muscifera, and 

 Ophrys aranifera, in every one of which the epider- 

 mal cells, on both the upper and under sides of the 

 leaf, were much alike and — save the stomata on the 

 under surface — resembling the same cells on the 

 upper side of the leaf of the Tway-blade. To define 

 the exact value of this character would require an 

 examination of the wilderness of exotic orchids as 

 well as all our native species. But the remarkable 

 character now described suggests a wide and 

 probably fertile field for future cultivation. At 

 present we know that, among the Duckweeds, 

 Lemna minor is easily distinguishable, by its sinuous 

 epidermal cells, from IFoIffia arrhiza, though these 

 two plants were formerly considered as identical. 



On the Crystals in the Seed-coat of the 

 Elm:-tree (JJlmus campestris). — At this season, or a 

 little earlier, the fruit of the Elm is shed and scat- 

 tered in profusion on the ground, often so as to 

 make patches in our paths. Each fruit is a capsule, 

 somewhat oval, vei'y flat, and about as big as the 

 thumb-nail. The seed is contained near the centre 

 of this compressed and winged capsule or samara, 

 and the outer coat of the seed is the seat of the 

 crystals. Every cell of this part contains a short 

 and brilliant crystal, in form cubical, lozenge-shaped, 

 or prismatic, and presenting a long diameter of 

 about l-2666th, and a short diameter of l-3555th of 

 an inch. They are beautiful microscopic objects, 

 and perhaps may be found well adapted for experi- 

 ments with polarized light. The crystals are com- 

 posed chiefly of oxalate of lime. 



Sacchako Polauiscope. — "W. L.'s" assertion 

 that the round bottom of the tube forming a plano- 

 convex lens would prevent any light reaching the 

 eye is simply absurd. How is it that an ordinary 

 "BuU's-Eye" allows the light to pass through it, 

 and illuminate an object far beyond the focus of the 



convex lenses would be useless, and refracting 

 microscopes and telescopes could never have been 

 made. If, instead of assuming a tone of superiority, 

 "W. L." had stated why a ray of light would not pass 

 through a plano-convex lens, he would have con- 

 ferred a benefit on " young people ignorant of 

 optics," and also upon myself. My experience has 

 been to the contrary. The following diagram will 



lens ? In fact, if " W. 



L.'s" 



assertion were true. 



Fig. 102. Diagram sliowing passage of light through 

 plano-convex lens. 



perhaps enable " VV. L." to understand how a ray of 

 light doespass through a plano-convex lens :—<?,«, a, 

 parallel rays of light converging to a focus at b, after- 

 wards forming the divergent rays c, c. Every one with 

 the least knowledge of optics is aware that the rays 

 of light are less refracted as they approach the 

 centre of the lens, and of course are less divergent. 

 There can be no reason why they should not enter 

 the eye at a considerable distance from the focal 

 point. The tube I use is about fourteen inches in 

 length and three-quarters of an inch in diameter, 

 and when filled with a solution of sugar allows a 

 brilliant beam of polarized light to reach the eye, 

 as " W. L." would have found if he had tried the ex- 

 periment, instead of assuming that I had not done 

 so. I am quite aware that a far superior arrange- 

 ment for the purpose required by "P. M. S." could 

 have been supplied by a first-class optician, but the 

 inquiry was for a cheap and simple form of instru- 

 ment. The tube of glass might also be replaced by 

 one of vulcanite, with diaphragms fitted in the 

 interior, and the wooden stand replaced by a metal 

 one ; — but simplicity and cheapness would have to 

 be sacrificed. — F. Kitton. 



ZOOLOGY. 



"The Saturday Half-Holiday Guide."— It 

 is with pleasure that we notice the cheap little 

 brochure bearing the above title, as we feel confi- 

 dent it is just the sort of thing often much wanted 

 by young naturalists. It gives a capital descrip- 

 tion of the parks, environs, sports, historical sites, 

 antiquities, &c., of the neighbourhood of London ; 

 but what is more to our point, it directs the natu- 

 ralist to the metropolitan natural history resorts, 

 and records the habitats of rare plants, the birds, 

 the best entomological resorts, the microscopical 

 collecting-grounds, the places where certain mosses. 



