CHAPTER X. THE THALLOPHYTA. 



333 



having a thallus of indefinite form adhering so closely to the 

 substratum on which they grow, that it is often difficult to distin- 

 guish them from it. They grow on rocks, smooth-barked trees, 

 wooden fences, and sometimes on the ground. One, called 







Fig. 538. Fig. 539. 



Fig. 536. — Portion of thallus of Sticta pulmonacea, a foliaceous Lichen, a, apothe- 

 cium. Natural size. 



Fig. 537. — Thallus of Umbilicaria vellea, a foliaceous Lichen, a, apothecium. Natural 

 size. 



Fig. 538. — Thallus of TJsnea barbata, a fruticose Lichen, about natural size, a, apoth- 

 ecium . 



Fig. 539. — Thallus of Roccella tinctoria, a fruticose Lichen, about natural size, a, 

 apothecium. 



Fig. 540 — Portion of Cetraria islandica, a fruticose Lichen with a flattened thallus. 

 About natural size. 



