44 GUIDE TO THE STUDY OF LICHENS. 



sembles the upper cortical layer, usually it is defi- 

 cient, and in a number of genera it is wholly wanting. 

 When wanting, its place is taken by a tissue of hyphae 

 which extend longitudinally. From the lower surface, 

 whether cortical or not, the rhizoids extend. These are 

 hyphae, either single or in groups, which grow verti- 

 cally downward into the substratum ; they sometimes 

 remain aerial and perhaps serve to retain moisture or 

 to keep off crawling insects and snails. Usually the 

 rhizoids are colored black. The cilia at the marsfin of 

 many foliose thalli are morphologically and function- 

 ally analogous to aerial rhizoids. (Plates II, III and 

 IV.) 



3. The Fruticose Type. 



The fruticose thallus differs from the preceding in 

 that the lobes of the thallus are distinctly ascending 

 and are not attached to the substratum. The entire- 

 plant is attached at a central or basal point known as 

 the umbilicus. It must, however, be remembered, that 

 the umbilicus is also present in some foliose thalli, as 

 Umbilicaria and Gyrophora. The umbilicus consists of 

 a hyphal tissue, and is analogous to the roots of trees, 

 holding the plant firmly attached to the substratum 

 and taking therefrom moisture and soluble food-sub-, 

 stances. 



The lobes of the thallus are usually much branched, 

 and vary from distinctly flattened to cylindrical. The 

 highest and most perfect type is, no doubt, represented 

 by Usnea harbata. 



In the flattened fruticose thallus we find the follow- 

 ing tissues, which are essentially like corresponding 



