INTRODUCTION 17 



asci very soon begin to enlarge so that they may be distinguished from those 

 producing the paraphyses while still very small. Also these ascus-producing hyphae 

 always remain non-septate and are much richer in protoplasm than the others. 

 Some observers have thought that the asci and paraphyses arise from different 

 systems of hyphae, and have called the supposed ascus-producing elements ascog- 

 enous hyphae, regarding them as arising from the ascogonia, but Sturgis arrived 

 at the conclusion that both asci and paraphyses arise from the same system of 

 hyphae. As the development proceeds, the hypothecium is produced into the 

 exciple or the perithecial wall, as the case may be. 



Beginning its development within the thallus, the ascocarp may remain im- 

 mersed or it may become more or less superficial, being supported in ways already 

 described. As the ascocarp pushes upward the surrounding thallus may or may not 

 grow up about it to form the thalloid exciple, and now, if at all, is formed the 

 epithecium. 



REPRODUCTION 



Soredia. Among the means of reproduction in lichens soredia play an impor- 

 tant part. These masses of hyphae and algae may in proper environment produce 

 a lichen thallus. They occur on many foliose and fruticose lichens and on some 

 crustose species. A whole soredium or a portion of one is easily carried by the 

 wind, and resting on a favorable substratum may develop into a lichen thallus 

 having the same structure and bearing the same kind of ascocarps, soredia, sperma- 

 gonia, etc., as that on which the soredium developed. 



There is no differentiation into layers in the soredium, nor is dorsiven- 

 trality established until after it begins to grow on the substratum. As growth 

 begins, the hyphal rhizoids penetrate the substratum to secure nourishment and 

 to anchor the minute mass of tissue. As growth proceeds, the hyphae usually 

 become differentiated into cortex and medulla, and the algae come to occupy their 

 proper place in the thallus. 



Soredia may develop on the thallus on which they are produced into isidioid 

 or coralloid branchlets. These are frequently found on the thalli of some of the 

 foliose forms. Doubtless the reproduction by soredia is the most important method 

 in many lichens. Indeed, a considerable number of the foliose lichens and a 

 few fruticose and crustose ones seldom produce apothecia and spores. 



Fragmentation. Another method of vegetative reproduction among lichens 

 is by fragments of thalli becoming detached and blowing away and growing upon 

 a suitable substratum. It does not matter how small the fragment provided it 

 contains both the algae and the hyphae. There may or may not be all the layers 

 of the thallus represented in the fragment. Sometimes the fragment is an isidioid 

 or coralloid branchlet and in other instances it is a large and conspicuous portion 

 of the thallus. The latter condition is well illustrated in Usnea longissima, long 

 masses of which are often seen in the northern states hanging over branches, 

 without any attachment whatever. In foliose and crustose lichens also the frag- 

 ments may be large, and even whole thalli may be torn loose by the wind and 

 transported to a new substratum and there grow. 



Rejuvenescence. In a considerable number of lichens the older portions of 

 the thallus die while the younger portions continue to grow. This condition is 

 quite common among species of Cladonia, where the basal portions of the podetia 

 die and the branching above continues. The branches thus become separated and 

 a number of individuals arise from one. Usually such an assemblage forms a 

 dense cluster, but the central and more raised portion of the cluster frequently 

 dies or is blown away, leaving a continuous or more or less broken ring of plants. 



