240 



METABOLISM 



Fungi and Phanerogams, nitrogen fixation is a proved fact, or, at least, 

 very probable. One of the Coniferae especially, Podocarpus, is known to 

 possess nodules on its roots. These nodules are modified lateral roots, whose 

 tissues have become filled with the hyphae of a fungus. Podocarpus cannot 

 be cultivated in the absence of this fungus, but Nobbe and Hiltner remark 

 (1899 a) that they were able to grow Podocarpus with perfe^^uccess for five 

 years in quartz sand, from which nitrogen was entirely aaWt. There can 

 be no doubt that this plant is able to fix atmospheric nitrogen, and it is ex- 

 tremely probable that the fungus plays an important part in the process. 

 More recently, Hiltner (1899) has studied Lolium temulentum, with which, 

 according to Vogl and Nestler, there is constantly to be observed associated 

 a fungus growth. Hiltner's observations render it probable that here also 

 fixation of nitrogen takes place, and it may be concluded that many parasitic 

 Fungi act in the same way. Hiltner thought that the luxuriant growth of 



many plants attacked by Fungi was evi- 

 dence in favour of his view, but Brefeld 

 (1902) has shown that this does not apply 

 to Ustilagineae, although it cannot be said 

 that that is the case with other Fungi. 



The so-called 'mycorhiza', a symbiotic 

 union of a fungus with phanerogamic roots, 

 is of wide occurrence, and first suggests it- 

 self to us in this relation. There are two 

 forms of this union, endotrophic and ecto- 

 trophic. The former has been long known, 

 in fact since the time when Schleiden 

 pointed out its occurrence in Neottia nidus 

 avis, and its widespread distribution has 

 been demonstrated by Frank (1887) and 

 ScHLiCHT {1889), especially in Orchidaceae, 

 Ericaceae and Epacridaceae. These Fungi, 

 which have as yet been very little investi- 

 gated from the systematic point of view, 

 enter the cells of the root, and increase 

 there without killing the cells of the host. 

 Neottia has been carefully investigated by 

 W. Magnus (1900). In this case the fungus 

 enters from without, and branches at 

 some distance from the epidermis, so 

 as to completely fill up a series of con- 

 centric layers of cells in the root and 

 rhizome. The cells in which the fungus lives do not all behave in the same 

 way. In certain cells the fungus grows vigorously, and on the death of the 

 protoplasm of the host, forms organs for the purpose of maintaining its exist- 

 ence over winter, and for the infection of newplants the following year. The 

 fungus is partly digested in other cells, on the other hand, and its abundant 

 proteid constituents go to the nourishment of the host, while the indigestible 

 portions are collected into a ball in the centre of the cell, and are there enclosed 

 by layers of cellulose. The same thing is seen in Psilotmn triquetrum (Shibata, 

 1902), only in this case the fungal host-cells and the digestive-cells are arranged 

 in no definite order. In the host-cells the fungus hyphae are confined to the 

 periphery of the cell, and the nucleus undergoes no alteration ; in the digested 

 cells, on the other hand, one finds (Fig. 44) a dense ball of hyphae, which, begin- 

 ning at one point, is gradually disorganized (7 and // Kl.), while at the same time 

 the nucleus increases greatly in size, and undergoes special internal altera- 



Fig. 44. Psilotiim triquetrum. I, Cell of 

 the root filled with fung;al hyphae; n, nucleus; 

 digestion of the fungus beginning at Kl\ X 210. 

 /V; nucleus, m, and digested fungus, Kl, in a 

 similar cell, x 400. Ill, Normal resting nucleus 

 from the rhizome of Psi/o/unt, x 625. /V, nucleus 

 of an infected cell, x 6a.s. V, Cell completely 

 digested by the fungus, A"/, the digested fungal 

 mass enclosed in a cellulose membrane. >< 240. 

 After Shibata (1902). 



