7 8 



throughout the entire wall as well as throughout the septa. The 

 spore-walls may also undergo other secondary changes ; in the 

 spores of Pertusaria peculiar thickenings of the endosporium are 

 noticeable, which, no doubt, are for mechanical protection as well as 

 to favor germination, the mechanical adaptation being required on 

 account of their large size ; germination of the spores begins at the 

 thin areas. In other lichen-spores the exosporium is highly gelatin- 

 ous, aiding them in adhering to the substratum in localities favorable 

 for germination. 



Oil globules are quite constantly present within the spores, and 

 doubtless serve some purpose in the processes of germination. In some 

 spores the plasmic contents are very coarsely granular, in others they 

 are apparently quite homogeneous. It must be borne in mind that 

 lichen-spores have undergone great structural as well as functional 

 changes during their phylogenetic development. The spore-walls 

 tend to become thinner and colorless ; the septa have become ir- 

 regular in occurrence and form, and the tendency is evidently toward 

 simple spores. Many of the higher lichens are constantly without 

 spores ; the causes of such retrogressive changes will be discussed 

 later. 



• IV. GROWTH OF ALGAE. 



With certain exceptions the fungal symbiont constitutes the prin- 

 cipal mass of the lichen and encloses the alga. In Efhebe ^puies- 

 cens, however, the alga (Sirosifhon fulvinatus') not only forms the 

 bulk of the lichen, but it directs and controls the method of growth. 

 In this lichen, growth proceeds from a single apical cell which forms 

 two or more cell-rows ; branching is distinctly dichotomous. The 

 hyphae rarely extend to the apex ; some branches may be entirely 

 free from them. 



In other lichen forms neither alga nor fungus seems to predomi- 

 nate ( Collema) but the symbionts appear to be about equally distrib- 

 uted. In this group of foliose lichens the alga is a species of Nos- 

 toc in most respects similar to Nostoc commune; it occurs in chains 

 loosely intermingled with the hyphae which do not form haustoria. 

 The principal growth is marginal, that is, new alga-chains with heter- 

 ocysts are continuously formed either from single cells or as branches 

 from older chains. Certain marginal areas grow more rapidly than 

 others, simulating apical growth and causing lobation or branching 



