34 



LECTURE III. 



^«^^ 



Fig. 25 — A portion of the foliaceous thalUis o 

 Pettigera horixotttalis, a apothecia ; r roots (nat. 

 size). 



merely water and dissolved salts, but also organic food-matters. Apart from certain 

 special cases of complex formation of tissue, the mycelium consists of much-branched 

 tubes, which are unsegmented in the Phycomycetes, but in all other cases divided by 

 transverse walls. From the manner in which these cell-filaments penetrate into the 

 substratum, become branched there, and, under certain circumstances, again come 

 forth from the substratum, and so on, it follows that the mycelium of the Fungus 

 agrees entirely with typical roots in respect to geotropism and heliotropism, and in 



its sensitiveness to moisture and contact; 

 The mycelium, it is true, possesses the 

 property of producing fruit-bearing hyphae, 

 which we have already distinguished^ as 

 the formation of the shoot in Fungi. 

 This is not an essential deviation from 

 the typical roots, since in many phane- 

 rogams and vascular cryptogams the 

 tendency exists in a very prominent dcr 

 gree to produce shoots out of roots; and 

 we meet further with the formation of 

 shoots out of roots in the Mosses. It 

 cannot therefore be urged against the 

 root-nature of the mycelium of the Fungi, 

 that the fruit-bearing portions Ordinarily 

 spring out of it as shoots. This happens 

 in fact even in Monotropa — a phanerogam 

 devoid of chlorophyll — in a quite similar 

 manner. 



Among the Fungi, the group of Lichens 

 is distinguished by a very remarkable 

 form of parasitism. The fungus tissue, in 

 fact, surrounds the Algae containing chloro- 

 phyll which nourish it, so that the latter 

 behave like a histological constituent of 

 the Fungus, which has now become in a 

 certain sense a plant containing chloro- 

 phyll. In accordance with this, the shoot- 

 formation of the Lichens is also often far 

 more complete than in other Fungi, and 

 thence follows, again, that in them the con- 

 trast between shoot and root is more sharply 

 expressed than in other Fungi. 

 Here too, again, the root may act not merely as an organ of attachment 

 only, but also at the same time as an organ of nutrition. The first is the case in 

 many so-called fruticose lichens, which are attached by a narrow base to the 

 dry bark of trees, e.g. the genus Usnea. The roots of the so-called foliaceous . 

 lichens, the shoots of which are extended as fiat dorsi-ventral plates on the earth, or 

 on tree trunks, as in the large genus PelHgera, appear not only as organs of attach- 



FIG. 26 —Usnea Sarbata, a fruticose licllL... 

 tification ; y organ of attacliment, which becpmes firmly fixed 

 on the bark of a tree. 



. ^ fruc- 



