I02 



HARDWICKE'S SCIENCE-GOSSIP. 



the stem was small. As the cones grew, they seem 

 to have had a depressing action on the leaves of the 

 stem, and thus to have formed the well-known scais 

 of ulodendron. Ilalonia and ulodendron are, there- 

 fore, both cone-bearing branches of Lepidodendron. 



Several species of lepidodendron are found in our 

 coal balls, the most common being Lepidodendron 

 selaginoides. L. Harcotirtii is less plentiful, but 

 Sigillaria vascularis (BintTey) or, as Professor W. 

 C. Williamson prefers to call it, Lepidodendron vascu- 

 lare, is moderately common. 



Lepidodendron selaginoides. The specimen from 

 which fig. 65, is taken is about half an inch in 

 diameter and is remarkably well preserved. All its 

 various tissues, with the exception of a small por- 

 tion of the inner bark, which is rarely met with, are 

 as perfect as they were when the plant was living. 

 The pith is formed of barred cells and vessels. Sur- 

 rounding this vascular axis is a small zone in which 

 are a number of vascular bundles proceeding from 

 the pith to the leaves. These bundles may be seen 

 here and there in the space beyond. This zone is 

 also occupied by very fine cellular tissue and bounded 

 by a layer of parenchyma, formed of rather larger 

 cells than those within the zone. From the latter 

 layer proceed in a somewhat radiating manner, a 

 number of cellular laminre, which break up into 

 regular parenchyma before reaching the periphery 

 of the space. These tissues are, in the great majority 

 of specimens, entirely absent, owing probably to their 

 very delicate nature, and even in the specimen now 

 being described, they are in places much shrunk and 

 shrivelled. Surrounding the pith there is a thin layer 

 of prosenchyma, composed of very small vessels, 

 which cannot be distinguished in transverse sections 

 from the rest of the small vessels composing the rim 

 of the pith, Ijut which can be plainly seen in the 

 longitudinal section. 



This is the layer from which proceed the vascular 

 bundles which go to the leaves. Surrounding this is 

 the delicate parenchyma, forming what Professor 

 Williamson calls the pseudo-cambial layer [g], and 

 forming the innermost layer of the inner bark. The 

 inner bark is seldom found preserved as in this speci- 

 men, there being generally a zone between the pith 

 and the middle bark filled with calcite and stigmarian 

 rootlets. The middle bark, which is almost always 

 preserved, is composed of rather large cellular tissue, 

 interspersed with the vascular bundles proceeding from 

 the pith to the leaves. These bundles were originally 

 surrounded by delicate parenchyma, the decay of which 

 has left spaces in the bark destitute of tissue. These 

 spaces have become filled with calcite during the pro- 

 cess of fossilisation, and form a conspicuous feature in 

 many sections of lepidodendroid plants. The large cells 

 of the middle bark gradually merge into the smaller 

 vessels of the bast layer. In most specimens the outer 

 portion of the bast layer has broken off from the inner 

 portion, and in this state the specimens are some- 



times said to be decorticated, whereas they have 

 simply lost their epidermal layer. We often find 

 portions of the epiderm in the material separate from 

 the other parts of the plant. Most of our common 

 lepidodendroid fossils found in sandstone, shale, &c., 

 are either casts of the epiderm or of some other part 

 of the bark. 



HOW TO CLEAN DIATOMS. 



IN the December (1S80)' part of the "American 

 Monthly Journal of Microscopy" Professor 

 H. L. Smith writes in glowing terms of the use of 

 soap in the preparation of diatoms, and describes his 

 method of procedure, which we give in his own 

 words. The material operated upon was a gathering of 

 marine diatoms from the Hawaiian Islands ; tliey 

 were dried and matted together, but the gathering 

 was rich in diatoms. " I put the dried material into 

 a large test tube and covered it well with nitric acid, 

 and left it for an hour (the time is not material). 

 After this I boiled it, adding a little more nitric acid, 

 and dropping in two little bits of bichromate of potash 

 again boiled for a minute or two. The tube was 

 now filled with rain-water and the whole mixed b)- 

 inverting, &c. As soon as the mass (llocculent enough) 

 had settled, the discoloured and acid water was poured 

 off, and the tube filled with rain-water (it is better 

 to use soft water). After settling, the water was again 

 poured off and the deposit once more washed. (So 

 far this is the old method, which generally ends here, 

 only washing out as best one can the lighter forms 

 and broken frustules.) . . . Pour off the supernatant 

 water and adding a little clean soft-water I now put 

 in a small piece of yellow soap the size of a pea and 

 again boiled the deposit for a minute or two, after 

 which the tube was filled with clean rain-water ; some 

 fifteen or twenty minutes after the yet milky fluid was 

 poured off (it contained but few and very minute 

 diatoms, which one can save if so disposed by keeping 

 the poured-off fluid for a longer time). The tube, 

 with the heavy deposit at the bottom, was again filled 

 with rain water and shaken, and now a brilliant, 

 sparkling display of colours showed that the flcc- 

 culent matter was gone, and only clean diatoms, 

 sponge spicules, and i^ossibly some sand remained. 

 Finally distilled water and alcohol was added, and 

 after this washing pouring off all the finer forms which 

 remained in suspension after five minutes, and ^^■hich 

 were saved with those of the other washings, litt'e 

 else remained except clean diatoms. 



If after this any one exhibits a slide of diatoms full 

 of flocculent and dirty deposit i:)eculiar to the old 

 method of treatment, we can only say, as in Miss 

 Edgeworth's story, " What, no soap ! " 



Professor Smith says at the commencement of his 

 paper that the use of soap for cleaning diatoms is not 

 a discovery of his own, but that he cannot remember 



