JlLV. 1911. 



KNOWLEDGE. 



277 



water and salts, and it shares with the Fungus in producing 

 the asexual brood bodies or soredia. 



Both the Fungus and the Algae which make up a 

 Lichen can. under suitable conditions, be induced to grow 

 independently, though in the Lichen body itself they are 

 mutually dependent upon each other. The isolated Alga 

 cells grow and multiply when suppUed with water, a few 

 simple salts, and air containing carbon dioxide. The Lichen 

 spores will germinate in a culture solution containing organic 

 substances (sugar, and so on) and produce a small thallus 

 which contains, of course, no Alga cells. 



The reproduction of a Lichen is somewhat complicated, for 

 we have to consider the processes of multiplication carried on 

 by (1) the Alga cells, (2) the Fungus, (3) the Lichen as a 

 whole. ( 1 1 The .^Iga cells increase in number by growing 

 and dividing as if they were living independently, but do not 

 bring about the reproduction of the Lichen as a whole. (2) In 

 spore formation the Fungus alone is concerned, the spores 

 being formed in special sac-like threads (ascil which develop 

 from the Fungus web. The spores on being carried by the 

 wind to a suitable place, germinate and, if the right .^Iga is at 

 hand, the Fungus threads surround them and a Lichen thallus 

 is woven. By sowing Lichen spores on plates on which 

 minute Algae are growing, we can readily observe the 

 production of a Lichen. In Nature, the .-Vlgae usually present 

 in Lichens are widespread, so that the formation of a young 

 Lichen thallus depends largely on the existence of suitable 

 general conditions for growth. (3i Many Lichens are largely 

 propagated by means of small brood-bodies (sorediai. budded 

 off by the thallus. Each soredium consists of a few Alga 

 cells, or only one. surrounded by a web of Fungus threads. 

 Sometimes these soredia are produced in definite clumps, but 

 more often they form a powdery layer sprinkled o\er the 

 thallus. 



It must be remembered that the view of the symbiosis, or 

 mutually beneficial partnership, relation between Fungus and 

 Alga applies strictly only to the Lichens that grow on bare 

 rocks and stones. In fact, even in these cases the Fungus pro- 

 ceeding from the germinating Lichen spore must be supplied 

 with some organic substances derived from decaying plant or 

 animal remains on the rock or stone. The Lichens that grow 

 on ordinary soil, and on trunks of trees, resemble ordinary 

 Fungi in being saprophytic — that is, they live at the expense 

 of decaying organic matter, so far as the Fungus constituent 

 is concerned. Moreover. man\- Lichens that grow on leaves 

 in the Tropics, as well as some that grow on trunks, are more 

 or less distinctly parasitic, drawing part, at least, of their 

 nourishment from the living cells with which they are in 

 contact by their underside or by their fixing and absorbing 

 organs (rhizines). 



.\s a matter of fact, \arious writers ha\e maintained that 

 the relation between the I'ungus and the Alga in a Lichen is 

 simply one of parasitism on the part of the Fungus. In some 

 cases, the Fungus threads have been seen to penetrate the Alga 

 cells instead of merely spinning an enclosing web around them, 

 and the presence of dead Alga cells in various Lichens has 

 suggested the view that the Fungus kills the Alga by means 

 of ferments. Possibly the Fungus makes use of the .\lga for 

 some time after its spores have germinated, and then proceeds 

 to live in the same way as an ordinary saprophytic Fungus 

 when on a substratum rich in organic matter. This view is 

 supported by the fact that Fungi which normally form part of 

 a Lichen have been found growing independently, and there 

 seems little doubt that in most Lichens the Fungus is capable 

 of absorbing organic food from the substratum and is not 

 dependent upon that which is manufactured by the Algae. 



From the structure of the Lichen thallus it is clear that the 

 Alga cells are placed in somewhat unfa\ourable conditions for 

 the making of food by carbon assimilation. They are not at 

 all well situated with reference to light and air, being covered b>' 

 the thick dense cortical tissue which often contains various 

 pigments and is thus darkened, while the compactness of the 

 overlying tissue makes difficult the access of atmospheric air 

 to the green cells. The latter difficulty is in many cases 

 obviated by the presence of special canals and slit-like rifts in 



the cortex. In some of the Parinclia species, which have been 

 carefully examined by Rosendahl ^Nova Acta Leop.-Carol. 

 Acad., 1907) the cortex, elsewhere thick and consisting of dense 

 tissue, shows at places a loose texture corresponding to the 

 lenticels in the cork of tree stems, while in those species which 

 have a thin and delicate cortex these lenticels do not occur. 

 That lack of light and air tend to restrict the grow^th of the 

 .Algae is further shown by the fact that the usually continuous 

 .\lga layer is often interrupted below the fruits of the Lichen, 

 and also below the places occupied by parasitic Fungi which 

 are sometimes found growing on the surface of the Lichen 

 thallus. 



An interesting line of research in the physiology of Lichens 

 is opened up by the striking results obtained by Treboux and 

 other workers, with reference to the nutrition of the simpler 

 Green Algae. It has been shown that many of these 

 unicellular Algae can utilize organic acids (acetic, lactic, 

 oxalic, and so on), as a source of carbon, light being 

 unnecessary for the process, and that among the simple forms 

 tPleurococciis and others), which can obtain food in this way, 

 the common Algae found in Lichens are included. These 

 Algae, w^hen supplied with organic acid solutions, can grow 

 and develop in complete darkness. Lindau iLichcnologischc 

 Uiitcraiichitngeii. 1S95), noticed in a Lichen iPyrcitiila 

 nitida^ that not only the Fungus threads but also those of the 

 Alga, penetrated the b.irk of the tree on w^hich the Lichen was 

 growing, although the Alga (in this case a filamentous one 

 belonging to the genus Trciitcpohlia^ is only loosely bound 

 up with the Fungus threads. 



Tobler suggests that the Alga in a Lichen may utilise as 

 its source of carbon organic acids which are formed as bye- 

 products in the nutrition of the Fungus. It has long been 

 known that oxalate of Ume frequently occurs in crystalline 

 masses and incrustations on the threads of the Fungus in 

 Lichens, as well as on the threads of ordinary Fungi. In 

 Lichens, the oxalate is never found associated with the .-Vlga 

 cells. Oxalate is also produced abundantly in cultures of 

 isolated Lichen Fungi, even in cases where the Lichen itself 

 is free from it. From his culture experiments, using the 

 common yellow Lichen, Parinclia parictina. Tobler finds 

 that (1) calcium oxalate is freely produced by the Lichen 

 Fungus grow^n on gelatine ; (2) on the other hand, developing 

 Lichen plants on the same substratum, arising from the 

 .addition of the Algae to the Fungus, produce no o.xalate ; (3) 

 fully de\eloped plants of this Lichen, growling on bark, contain 

 no oxalate ; (41 in fluid cultures, containing no source of 

 carbon excepting the carbon dioxide of the atmosphere, the 

 Alga cells remain green and grow in the normal manner ; 

 (5) if the Lichen Fungus is also present in the culture the 

 .\lgae become colourless, but continue to grow luxuriantly. 



Many other facts support the view that the .\lga in a Lichen 

 may use the oxalic acid, and possibly other organic acids, 

 produced by the Fungus. For instance, Rosendahl showed 

 that in the brown Paruielias, thinness of the cortex and 

 presence of oxalate of lime go hand in hand — evidently in 

 such cases the .\lga receives sufficient light to enable it to 

 make food by photosynthesis, hence it does not use up the 

 oxalate. 



EVOLUTION' OF THE FLOWER.— Wernham. in his 

 second article on Floral Evolution, in the Xexc Pliytologist, 

 (see ■' Knowledge " for June, 19111, deals with the Lower 

 Dicotyledons (Archichlamydeae) in some detail, with 

 special reference to their phylogenetic relations to the Higher 

 Dicotyledons (Sympetalae or Gamopetalae). It is remark- 

 able that whereas the flowers of barely twenty per cent, of 

 the former have the stamens equal to or less than the corolla 

 segments, this character is found in nearly ninety-five per cent, 

 of the species of Sympetalae. Again, only about eighteen per 

 cent, of the .\rchichlamydeae have a pistil composed of two 

 syncarpous carpels, while in the Sympetalae fully seventy-five 

 per cent, of the species have a bicarpellary pistil. The conclusion 

 to be drawn is that the progressive tendency to economy of 

 production, observable at work in the Archichlamydeae. has 

 reached a high degree of realization in the Sympetalae, in 



