16 LICHEN FLORA OF THE UNITED STATES 



found in the muriform condition, though the others are probably to be regarded 

 as immature. In the genera of Teloschistaceae and Caloplacaceae the spores are 

 usually 1 -septate, but with a very thick septum through which a narrow canal 

 joins the small cells placed one at each end of the spore. It might be argued that 

 this spore is not truly septate, as a dividing wall across the connecting canal is very 

 rarely seen. 



In color the spores vary from hyaline to a blackish brown. Simple spores are 

 usually hyaline, but there are exceptions to the rule. Compound spores are very 

 frequently more or less colored. It is sometimes very difficult to decide the color 

 of spores. Some spores become brown or tinged brown very tardily, sometimes 

 not until shriveled out of recognition and probably dead. These are sometimes 

 seen in the same hymenium with normal spores which are hyaline. In such in- 

 stances, the spores should be called hyaline. Finally, aside from these exceptions 

 there are instances in which the beginning student will not be able to decide 

 whether spores which show a slight tinge of color should be called hyaline, nor 

 does the experienced worker decide correctly in all instances. 



Thecial Algae. In some lichens there are found in the hymenium certain 

 algal cells, commonly known as thecial or hymenial algae. These algae are usually 

 smaller than those of the thallus, and are perhaps of the same species, varying 

 in size because of a difference in nutrition; but it is by no means certain that the 

 thecial algae are derived from those of the thallus of the lichen in which they occur. 

 They may be looked for especially in mature plants of Dermatocarpon and Endo- 

 carpon, though they may occur in some other closely related genera. The algal 

 cells are usually smaller than those of the thallus. and are often found clinging 

 to the asci or to the paraphyses. By some workers it is supposed that they are 

 dispersed with the spores and are at hand when the spore germinates, so that the 

 parasitic relation may be established at once and a thallus readily built up, pro- 

 vided other conditions are favorable. However, it may well be doubted whether 

 reproduction often takes place in this way in nature, and it is much more prob- 

 able that the main function of these algal cells is to nourish the tissues within the 

 ascocarp. The fact that thecial algae are more common in immature than in 

 mature ascocarps would favor this view. They are found in many ascocarps of 

 other lichen genera, but seldom persist. Doubtless these non-persisting thecial 

 algae at least are foreign organisms which, having gained access to the young 

 ascocarps, endure for a time under more or less unfavorable conditions. 



Development of the Ascocarp. The origin and development seem to be 

 very similar to that in the Ascomycetes in general. The first thing to be observed 

 in the development of an ascocarp is a mass of closely interwoven hyphae below 

 the algal zone, in the medullary area — this, of course, in those lichens in which 

 there is a distinct differentiation into layers. In species of Collema and some 

 related types, this differentiation is wanting, but the development here also 

 begins within the thalloid body. Even in the crustose thalli which show very little 

 differentiation, the first evidence of the ascocarp appears as more closely inter- 

 woven hyphae than those of the surrounding thallus. i 



This mass of hyphae constitutes the beginning of the hypothecium, and the 

 structure increases rapidly in size, spreading out laterally and also increasing in 

 thickness. From the hyphae of this young hypothecium arise vertical hyphae, 

 which soon become differentiated into paraphyses and asci, appearing first over 

 the central portions of the hyphal mass and extending laterally in all directions 

 as the hypothecium spreads out by lateral growth. The paraphyses and the asci 

 are very similar in their early development, but the hyphae which produce the 



