376 



NA TURE 



{Feb. 19, 1 1 



at two sides of the mountain, that merely proves that the 

 light was too feeble to penetrate a distance of "2' '45 km., though it was 

 visible at 1 '56 km. The reflection of the moonlight was also 

 stronger at point o than at III., because on this occasion the 

 moon was nearly north east. 



It is not easy, I confess, to make a risumi of the results 

 arrived at by the researches of the Finnish Expedition to Lap- 

 land concerning the electric currents of the earth and the atmo- 

 sphere, chiefly owing to the circumstance that the materials are 

 not as yet fully analysed, but the following may, however, be 

 accepted as quite certain, as they are based on actual observa- 

 tions : — 



The aurora borealis, which has long been a disputed enigma, 

 is the result of an atmospheric electric current. 



This auroral current can be measured, and, as a rule, studied, 

 by the methods employed by the Expedition. 



The " discharging-apparatus," or network of pointed con- 

 ductors, used by the Expedition, has very often produced a 

 diffuse light which gave in the spectroscope an auroral spectrum. 

 Under very favourable conditions the light appeared in the 

 form of rays above the apparatus. 



With a Holtz electric machine the diffuse light may be pro- 

 duced under favourable conditions, and if it exists already it may 

 be considerably intensified by the same means. 



For the study of terrestrial electric currents a method has 

 been found which, while avoiding all foreign influences, permits 

 of the current being measured, both as regards absolute strength 

 and as regards the exact laws which regulate it. 



From these experiments it seems that the existence of a belt of 

 terrestrial currents similar to the belt of auroral currents is very 

 probable. Selim Lemstrom 



Helsingfors University 



ON THE NATURE OF LICHENS 



TN the Journal of the Linnean Society for December 12, 

 1S84 (Botany) there appears a review of the " Algo- Lichen 

 Hypothesis," by the Rev. J. M. Crombie, F.L.S., from the 

 strongly conservative point of view of Nylander, on which I 

 desire to make a few remarks as a critical student of Botany at 

 large. 



Mr. Crombie cites, as a fatal objection to Schwendener's 

 hypothesis of symbiosis between the lichen proper and the alga 

 forming its gonidia, that in other cases of vegetable parasitism 

 " the hosts usually become speedily exhausted and finally perish, 

 often involving in their death that of the parasite itself ; " whereas 

 here we have "a parasite exceeding in size and number of cells by 

 many hundred times the nourishing plant which it invests." It 

 is now over six years since I sent you, with reference to this very 

 point, a brief note on the subject, which probably escaped Mr. 

 Crombie's notice by its brevity, and of which I reproduce the 

 substance. The essential elements of nutrition of a fungus, so 

 far as we can judge from culture experiments, are as follows : — 

 (1) ash constituents ; (2) nitrogen in the form of nitrates, nitrites, 

 or ammonia ; (3) carbon and hydrogen combined in the form of 

 tartrate, carbhydrate, or fat, &c. Art alga requires only Nos. 1 

 and 2, deriving No. 3 by assimilation from the carbon dioxide 

 of the atmosphere and water. The lichen hyphae, aided by 

 excretion of carbon dioxide, can dissolve the ash constituents, 

 No. I, from the substratum, taking them up by the rhizoids ; 

 the rain probably brings No. 2 in the form of traces of nitrates ; 

 No. 3 can only be formed by assimilation in the algal part orgonidia 

 of the lichen. But, to obtain the carbhydrates, No. 3, there is no 

 need for the hypha to penetrate the gonidium or to molest its 

 protoplasm, as the algal cells have a cellulose wall, of which 

 the outer layers undergo constant gelification and renewal. Into 

 this it is that, as shown by Bornet (" Sur les Gonidies des Lichens, " 

 Ann. Sc. Nat. Bot., ser. 5, xvii.) the hyphae penetrate: and if 

 they only lived on this, once formed, there would be no strain 

 whatever on the resources of the alga. But, even if they stimu- 

 late an abnormally rapid cellulose formation, the injury need 

 not necessarily be severe. We see oysters living well, though 

 their shells are burrowed by the sponge Cliona ; they produce 

 new layers of shells far faster than when they are sound, but are 

 otherwise uninjured. 



An unlooked-for confirmation of these views is found in 

 Johow's account of the Hymenolichenes (in Pringsheim's jahr- 

 bucher, xv., part 2), where, " in consequence of the unusually 

 close and perfect investment of the gonidia" by the hypha?, the 



gelatinous investment of their cell-wall completely disappears. 

 This is in marked contrast with the usual state of things as 

 figured by Bornet. 



De Bary puts the case thu 5 : — "With their growth (of the 

 algae) the assimilation of carbon dioxide persists in their proto- 

 plasm with its chlorophyll, and produces organic carbon com- 

 pounds utilisable by the fungus. At the same time the rhizoids 

 of the fungus ramify on and in the substratum, and bring the 

 mineral pabulum required. These two processes support and 

 complement one another ( Vergleichende Morphologic u. Physio- 

 logic d. Pilze, &c, 1884, p. 425). 



It is further noteworthy that, if the growth in size of the 

 gonidia is often favoured by their inclosure in the lichen-thallus, 

 their rapidity of multiplication by division is notably impeded ; 

 while spore-formation , &c. , remains in complete abeyance. 



Mr. Crombie recalls the absence of algae in places where 

 lichens abound, e.g. "granitic detritus and boulders towards the 

 summit of lofty mountains." This follows from the fact that the 

 algae alone cannot there obtain, unassisted, their papulum No. 1, 

 the mineral substances or ash constituents. The absence of the 

 fungi alone from these localities simply shows that they cannot 

 live without their algal gonidia. 



Mr. Crombie gives as an essential distinction between the 

 hypha? of lichens and those of fungi the character of their cell- 

 wall : "perennial, firm, penetrated by lichenin, thick, im- 

 putable, and insoluble in caustic potash in the former ; caducous, 

 very soft, with thin walls, readily putrifying on maceration, and, 

 on the application of caustic potash, immediately becoming 

 dissolved." 



As regards the thickness and permanence of the walls, it 

 needs hardly to be recalled how much this character varies in 

 different parts of the same fungus, and notably in corresponding 

 organs of different members of the same group of fungi : com- 

 pare Polyporus and Boletus, Schissophyllum and Cop/inns. As 

 to the presence of lichenin, De Bary states (op. at., p. 10) 

 that in at least three gelatinous fungi — Hyduum erinaceus, 

 Polystigma, and Hys/erium macrosporium — the hypha turns blue 

 on the application of aqueous solution of iodine, that is, gives the 

 " lichenin reaction." 



As regards the alleged solubility of fungus hyphae in caustic- 

 potash, I am at a loss to understand it, having, like most workers, 

 been in the habit of using this reaction "for clearing" vegetable 

 preparations to demonstrate the presence of parasitic fungus 

 hypha?, which would be impossible if it dissolved them. And I 

 find no account of this solubility of fungal cell-walls in Hof- 

 meister's very complete " Lehre von der Pftanzcnzelle," or in 

 De Bary's above-cited work. 



A misapprehension on the part of the author is to think that 

 the Schwendenerian school have overlooked the " cellular 

 cortical layer " when they speak " as if only two elements entered 

 into the structure of lichens, viz. hyphae and gonidia." This is 

 due, so far as it is true, to the general recognition by mycologists 

 that such pseudo- parenchyma as that composing the cellular 

 cortical layer of lichens, of fungus sclerotia, &c, is only an 

 extreme modification of the hyphae. But, far from being ignored, 

 it is figured and described by Sachs ("Text-Book of Botany," 

 (1st Engl, ed., Figs. 188, 189, and explanation), who says: 

 "The upper and under cortical layers [of Sticta] also consist of 

 hyphae, which, however, . . . consist of shorter cells, and arc 

 united without interstices, forming a pseudo-parenchyma." Its 

 formation is also described by Bornet (op. cil., p. 97), and De 

 Bary writes (pp. eit., p. 436): "The hypha-branches forming 

 the cortical layer ('Rindenschicht') are united 

 without interstices, save in certain species of Roeella. They are 

 either recognisable as such, having the lumina of their segment- 

 cells evidently elongate 1 and cylindrical, even though shorter 

 than those of the medulla, or else they are formed of short iso- 

 diametric rounded prismatic cells, giving the cortex the structure 

 of a pseudo-parenchyma, which is often extremely regular and 

 neat (' zierlieh'). . . . The structure of these cortical layers, 

 shows great similarity to that of many sclerotia.'' 



In the latter half of the paper Mr. Crombie exposes at length 

 the view that the gonidia originate in the cellules of the hypo 

 thalline and cortical layers, 1 and illustrates it by figures. In 

 this no attempt is made to show the part played by the proto- 

 plasm in the process, an omission which is an implied confession 

 of the inadequacy of the weapons, optical and technical, 



1 As regards his statement that ''specimens illustrating the earlier stages 

 o r lichen growth appear to be unknown to the supporters of Schwenden- 

 ■y to revert to Bornet's paper, p. 97. 



