OBLLULARES, OR CELLULAR PLANTS. 61 



outer bark of the lichen nearly covers the apotheoia thus im- 

 mersed, and air and light are admitted only through a small 

 aperture called an ostiole (ps, oris, a mouth). In other spe- 

 cies of pulverulent lichens, the fructification reaches the sur- 

 face, and is seen in the form of a dark convex nucleus of cells. 

 In the crustaceous and foliaceous lichens, the fructification 

 rests on the surface, or is situated on the margin of the ' thal- 

 lus in saucer-like bodies or shields, the cortical matter of the 

 thallus forming a border round the nucleus. 



70. In the higher developments of lichens, the thallus 

 rises into a kind of axis, the cortical matter forming a pode- 

 tium or stalklike process containing the apothecia at its sum- 

 mit; or it assumes an arborescent appearance, as in Cladonia 

 rangeferina, the reindeer-moss, or in Usnea, the beard-moss, 

 which at first grows upright, but afterwards becomes pendu- 

 lous with age, and forms those long gray tufts of vegetable 

 matter which hang from the boughs of trees. In these 

 instances, the fructification is contained in little circular 

 shields at the extremities of the branches. 



71. The annexed illustrations, from the Botanical Text- 

 Booh of Dr. A. Grray, convey a very good idea of the organi- 

 zation of the different species of lichens. Pig. 27, 1, is a 

 stone covered with several species of pulverulent, crustaceous, 

 and foliaceous lichens. In the centre of this stone, we have 

 Sticta miniata, a lichen whose apothecia are immersed in 

 the substance of the thallus; 2 is a piece of its thallus 

 magnified, in which the ostioles and immersed apotheoia are 

 seen more distinctly. On the right-hand margin of the stone, 

 we have crustaceous lichens, the fructification being visible on 



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