6i 



3. ]n the majority of tlie higher foliose and fruticose lichens the 

 rhizoids are no longer primarily adapted to perform the function of 

 absorption, as is evident from their structure. They do not consist of 

 a network or threads of hyphal branches, but occur in bundles of 

 elongated closely adnate hyphal cells, thus forming also a mechanical 

 tissue specially adapted to resist longitudinal pulling tensions. 

 That this mechanical function is highly important in the larger 

 foliose and fruticose thalli is very evident, but the function of absorb- 

 ing and conducting food substances is likewise important ; it is diffi- 

 cult to determine in many instances which function is the more im- 

 portant. Sometimes the rhizoidal bundles are simple, as in many 

 Parmelias and Physcias ; frequently they are branched, as in the 

 umbilicus of Umbilicaria and other genera and in the rhizoids of 

 Gyropkora and some Parmelias. The so-called umbilicus is a very 

 much enlarged branching rhizoid ; microscopically it has a typical 

 root-like appearance. 



Sometimes these rhizoids are likewise aerial and serve to retain 

 moisture as well as to form a suitable nidus for the growth of certain 

 algae (see Contingent Symbiosis). In nearly all cases the rhizoids 

 are colored differently from the thallus, usually black ; as to the 

 purpose of this coloring substance, which is deposited in the cell-walls, 

 there is no satisfactory explanation given ; it may be possible that it 

 has some influence upon sunlight and the absorption of warmth. 

 According to Zukal the coloring matter in the rhizoids, and elsewhere, 

 forms a protection against the attacks of animals (snails) on account 

 of its disagreeable bitter and acrid nature, due to the presence of 

 lichen-acids ; according to the same authority the aerial rhizoids also 

 serve to keep away crawling animals. 



7. THE CILIA. 



Cilia are slender, rhizoid-like structures which normally occur 

 along the margin of foliose and flattened fruticose thalli ; morpho- 

 logically they resemble the more highly developed rhizoids, from 

 which they are, no doubt, phylogenetically derived. They consist of 

 bundles of hyphal cells, either simple or branched, and are usualhy 

 black or dark-brown, due to a coloring substance deposited in the 

 cell-walls ; some are ash gray, as in those of Physcia stellaris. 

 Structurally they are almost identical with the rhizoids ; they are, in 

 fact, modified marginal rhizoids, which have assumed an entirely 



