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 Algse 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 hyphe. 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 
hyph® 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 Nostocine®, appears as one 
member of the partnership; whilst the second is some species of Leptothrie or 
Hypheothriz. The green moniliform rows of cells of Nostocine are enmeshed 
and wrapped round by the delicate, filamentous cells devoid of chlorophyll of the 
Leptothrie 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 eases the filaments destitute of chlorophyll 
rest upon the moderately thickened cell-membranes of the green alg, 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. 
