33 



which extend parallel with the longitudinal axis ; it is enclosed in 

 Usnea by a tissue consisting of much modified, slightly colored 

 hyphal cells, which may be termed a protective sheath analogous 

 to the endodermis of vascular bundles in higher plants. 



Under this form of mechanical tissue we must also include the 

 elongated tissue-bundles met with in foliose thalli ; as for example 

 the "veins" near the lower surface of the thallus of Hydrothyria 

 venosa and Peltigera venosa as well as the elongated hyphal groups 

 occurring in different parts of foliose and fruticose thalli. From 

 their structure and position it is evident that they serve to resist pul- 

 ling tensions. It is, however, very evident that many fruticose thalli 

 with solid cylinders are almost wholly subject to lateral tensions, as 

 Stereocaulon and Sfhaerofhorus ; this is explainable by the fact 

 that more conducting elements are required than could be supplied 

 by a thin hollow cylinder, and it is therefore functionally com- 

 parable to the solid tree-trunk. The same may be said of the stipes. 



Type III. Direct Guys : Peltigera afhthosa. — Among the foliose 

 thalli the cortical layers are not sufficient to give the necessary me- 

 chanical support and there must therefore be other adaptations to meet 

 this requirement. This we find in the rhizoids which act as direct 

 guys fastening the thallus to the substratum. Peltigera afhthosa is 

 selected as the typical example because in this lichen the mechanical 

 function of the rhizoids seems to have reached its highest develop- 

 ment. All rhizoids (excepting the aerial ones) must, however, 

 be included, although their prime function may be that of taking up 

 soluble food-substances. Their mechanical significance is especially 

 apparent in the higher foliose lichens, as Physcia, Parmelia and 

 Mallotium. 



The so-called "umbilicus" is a highly specialized mechanical 

 adaptation belonging to this type, occurring typically in Gyro- 

 phora and Umbilicaria . From some point near the middle of the 

 lower surface of the expanded thallus bundles of root-like hyphae 

 extend by means of which the lichen is quite firmly attached to the 

 substratum. The analogy, both as to structure and function, be- 

 tween the umbilicus and the roots of higher plants is striking, and will 

 impress itself upon the student without further explanation. It must, 

 however, be remembered that the homologue of the umbilicus is 

 the rhizoid, and that it is phylogenetically derived from the latter 

 structure ; whether the mechanical function predominates over the 

 physiological or not is undetermined. 



