HARDWICKE'S SCIENCE-GOSSIF. 



159 



summits, and when only three spores are seen, it is 

 that the fourth is concealed by the others. If this is 

 the case, then the name Tetrasporites should replace 

 Triplosporites. It has been shown by M. Brongniart 

 that the microspores are confined to the upper and 

 middle part of the cone, whilst the lower portion 

 bears simple spherical spores of a much larger size 

 (macrospores), a similar arrangement to that which 

 prevails in the existing Selaginellas. The macro- 

 spores (like the microspores) are also found, according 

 to Schimper, in groups of four. 



The Lepidostrobus differs from the above cone in 

 that it contains only the microspores, but closely 

 resembles it in the other respects. 



A third kind of cone has been called Flemingites. 

 In this each leaf-scale bears several sporangia in a 

 double row, instead of a single one, and in these again 

 the spores are all microspores ; Schimper however 

 doubts much whether the so-called sporangia are not 

 macrospores. Thus we see that whilst the first 

 described sort of cone resembles that of a Selaginella, 

 the others are very similar to the fruit of a Lycopo- 

 dium ; and the only marked difference presented by the 

 Lepidodendra when compared with the Lycopodiacere 

 is that their stems are rather more highly organised, 

 which is only what might be expected in an 

 arborescent form ; they have, as has been shown, a 

 zone of thin walled cells surrounding the woody 

 cylinder, and this by comparison with a similar 

 structure found in Isoetes (quill-wort) is recognised 

 as a true cambium layer, and the growth of the stem 

 increased somewhat after the manner of exogenous 

 plants. In fact Professor Williamson describes the 

 Lepidodendra as having an imperfect exogenous 

 structure, in which something like medullary rays 

 are seen ; the evidence, however, is by no means 

 quite clear as yet upon this point. 



Schimper has described as many as fifty-nine 

 species of Lepidodendra, besides many plants closely 

 related to them ; amongst these is the Ulodendron, 

 this was not dichotomous as were the Lepidodendra, 

 but only slightly branched near the summit ; its trunk 

 also was conical, and at any rate in the fossils much 

 fissured. The genus Knorria may be referred to the 

 same order j in this plant the leaf-bases on the stems 

 are very long and scale-like, and by this it may be 

 distinguished from Lepidodendron. Halonia is 

 another form which is characterised by tubercles, and 

 may have been the fruit-bearing branch of a Lepido- 

 dendron or possibly of Ulodendron. 



[To be conihrned.) 



Red Dead-nettle. — Is it not unusual for the 

 red dead-nettle (Lamiiifii piirpiiretirti) to produce 

 white blossoms ? The other day I found a patch of 

 this plant in which the flowers of about half were 

 pure white. I can find no mention of this variation 

 in any of the Floras I have. — T. C, Royle. 



MICROSCOPY. 



What is the meaning of the Sign x ? — I can 

 assure your correspondent E. Holmes that I do not 

 feel in the least "confused "on the subject of my 

 last communication ; and I think if he had more care- 

 fully considered my statement he would have admitted 

 its correctness. Let me try further to explain. I 

 maintain that the idea of amplification alone does 

 not cover the meaning of the sign X as used micro- 

 scopically. I took it for granted that it would be 

 understood that I spoke of this sign, not algebraically 

 or mechanically, but microscopically. Now let us 

 refer to what I take to be the standard by which the 

 sign X is to be defined. If we turn to any optician's 

 table giving the magnifying power of objectives — (say, 

 e.g. Pvoss's) we find a fg and A eye-piece described as 

 magnifying 500 diam. : I look at an object with the 

 above power, and I see it enlarged, and having many 

 markings clearly brought out. This I understand to 

 represent that object perfectly as X 500 diam. If this 

 is a correct standard, then the same object seen through 

 Ross's one-inch and A eye-piece magnifying 50 diam., 

 though increased in size ten times, is not X 500 diam. j 

 inasmuch as the representation is by no means the 

 same. But if this latter is the correct representation 

 of an object X 500 diam. then the former is not. 

 Hence, preferring to abide by the standard recognised 

 by opticians and microscopists generally, I repeat 

 the statement with which I closed my former com- 

 munication, viz. that (to my mind at least) "unless 

 there be detail corresponding with the amplitude, the 

 object is not x so many diam." Inferior objectives 

 of the same supposed magnifying power as Ross's 

 I'gth will not of course define detail as clearly, but by 

 just so much as they fail to reach the standard, they 

 fail to X 500 diam. — T. R. J. 



Observations on Living Organisms. — Consi- 

 derable interest attaches to the continuous observa- 

 tion under the microscope of some of the minuter 

 forms of rotifers and other animalcula, but there is 

 some difficulty with very small and active organisms 

 in the constant supply of the very limited quantity of 

 water we can use. I should be glad to know of the 

 means adopted by those who have been successful in 

 the study. The film of water has to be thin to allow 

 of the use of a high power, and limited in diameter, 

 or the organism escapes us. I find a very fine fibre 

 of cotton will convey too much and float the cover, 

 and also produce currents of too great force. — E. 

 Holmes. 



Sections of Coal. — I should like another word 

 on this subject, in reply to the four paragraphs in this 

 month's Science-Gossip. Mr. Lord says his method 

 of making coal sections is "easy and successful," 

 but he immediately states that it is only a particular 

 kind of coal that he has tried, and goes on to say that 



