Oak Tree Trunks and their Vestments 489 



is a very variable species, the source of much perplexity 

 to young moss collectors. Sometimes it is erect, sometimes 

 creeping. The pronounced curling inwards of the terminal 

 leaves is its chief characteristic. When growing on the 

 ground it is often larger. Here is some in " fruit." Take the 

 capsule and detach its lid, probably your pocket lens is not 

 sufficiently powerful to enable you to see the beautiful fringe 

 of teeth guarding the spores within. Put a fruiting piece 

 into your vasculum to examine under the microscope this 

 evening. Excepting on the north side, where patches of more 

 or less clean bark are observable, the whole of the remainder 

 are covered with lichens. As remarked above, being epiphytes, 

 these lichens do no harm to the tree, though it is possible that 

 in rare cases where trees are very thickly matted with lichens 

 growth may be hindered. How did they come ? First allow 

 me to direct your attention to a greenish film which occurs 

 near the base of the trunk on one of the exposed patches 

 of bark. Similar films may be seen on old gates, posts 

 and rails after rain. It is the common green algal, 1 a lowly 

 plant, consisting of rounded cells ever growing and ever 

 dividing in moisture. It arose from spores brought in the 

 atmosphere, for there is no spontaneous generation. If the 

 spores of certain lichen-fungi come into contact with some 

 of these algal cells, the latter do not continue to develop as 

 before. The fungus spores germinate upon and surround 

 them, protecting them as it were. Though we have spoken 

 of the fungus element as parasitic upon the algal part of a 

 lichen, yet it is not a true parasite. It does not harm and ulti- 

 mately kill the algal, but it is only a predominating partner, 

 which demands from the algal the organic food that it 

 requires, and in return supplies the algal with the dissolved 

 salts necessary for its existence. This is one of the most 

 marvellous facts in the plant world. Those without a know- 

 ledge of botany still consider lichens, as the old lichenists 



1 Protococcus vulgaris. 



