248 



LICHENS. 



of the compass was effected on this little marble column! It is necessary to add, 

 however, that lichens growing on stone, bark, or any situation of the kind do 

 not in all cases owe their original appearance on the substratum to a fresh union 

 of Alga3 and Fungi, but that there is a second mode of distribution of lichens. This 

 method consists in the transportation by air-currents of already completed social 

 colonies to places often situated at a great distance from the spots where the 

 initial union between Alga and Fungus was contracted. The process is as follows: 



in the interior of an old, large, and fully developed lichen-thallus certain groups 



of cells separate from the rest, each group consisting of one or more green algal 

 cells enmeshed in a dense weft of hyphae. When a sufficient number of these 

 daughter-associations has been formed the thallus of the parent lichen is ruptured 

 and the little miniature social-groups, which are termed "soredia", come to the 

 surface. To the naked eye a single soredium is only visible as a bright dot, but 

 all together they have the appearance of a mass of powder or meal lying loosely 

 upon the old lichen-thallus. In dry weather this mealy efflorescence is easily 

 blown away with other organic particles. If, then, a soredium thus removed 

 comes to rest in the crack of a rock or on any suitable substratum, the alga and 

 hyphae composing it continue to develop, and the organism grows into a larger 

 lichen-thallus, which is able to repeat the process just described. In regions where 

 lichens abound, soredia of the kind are found regularly amongst the elements 

 of the organic dust, and occur, indeed, mixed with fungal spores and algal cells, 

 so that it certainly happens not infrequently that two spots close together in the 

 same cranny of stone exhibit both sorts of lichen-growth, the one newly produced 

 by the concurrence and union of algal and fungal cells, the other a daughter- 

 association which has arisen from an old lichen, as a soredium, and is continuing 

 its development. 



Another case of symbiosis allied to that of lichens is manifested by certain 

 Cryptogams which live socially together under water and have received the 

 systematic names of Mastichonema, Dasyactis, Enactis, &c. In them also a plant 

 containing chlorophyll, and belonging to the group of Nostocineae, appears as one 

 member of the partnership; whilst the second is some species of Leptothrix or 

 Hypheothrix. The green moniliform rows of cells of Nostocineae are enmeshed 

 and wrapped round by the delicate, filamentous cells devoid of chlorophyll of the 

 Leptothrix or Hypheothrix; and later, by repeated processes of division, whole 

 colonies of green cell-filaments ensheathed in this manner are produced, which 

 to the naked eye appear as small soft tufts, usually clinging to porous limestone 

 in the spray of waterfalls. In many cases the filaments destitute of chlorophyll 

 rest upon the moderately thickened cell-membranes of the green algae, whilst 

 in other cases they insinuate themselves into the thick cell-membranes, permeate 

 them with their webs, and form in conjunction with them the sheathing envelope. 



