INTRODUCTION 15 



The Paraphyses. These structures are specialized hyphae which arise from 

 the tissues of the hypothecium. They are commonly cylindrical in form and 

 divided by transverse septa into a number of cells. They appear on hasty exam- 

 ination to be uniformly simple in most lichen species, but more thorough observa- 

 tion usually brings to light some that are branched in nearly every hymenium. The 

 branching may be very limited, or it may be extensive, as in most of the species 

 of Arthoniaceae. Statements as to the branching are included in the description 

 of each genus. The paraphyses are usually distinct, but sometimes their walls 

 are more or less gelatinized, forming a structure in some degree coherent and 

 indistinct. They are usually longer than the asci, but are shorter in the species 

 of Verrucaria, in which they also become imperfectly or often wholly gelatinized. 

 The tips, or apices, are usually thickened and darker in color than the remaining 

 part. Other portions of the paraphysis may be somewhat colored; but usually the 

 single organ appears quite hyaline, though the section of the hymenium often shows 

 color. The paraphyses serve to protect the asci and the contained spores against 

 too rapid transpiration and aid in the dispersion of the spores. The thickening 

 and coloration of the apices are protective in function. 



The Asci. The asci arise, as do the paraphyses, from the tissue of the hypo- 

 thecium. They are usually shorter and wider than the paraphyses which surround 

 them. In form they are most commonly clavate, but they may be cylindrical, pyri- 

 form, subglobose, or variously ventricose, or otherwise irregular. The walls are 

 usually more or less thickened toward the apex, probably by an accumulation of 

 epiplasm. The thickening may be very slight or it may occupy the upper third 

 or more of the ascus, as in some of the Arthoniaceae. There is a succession of 

 asci produced in each ascocarp, and one may rarely find asci of two generations 

 together, those of one generation containing mature or perhaps old and shriveled 

 spores, and those of the other younger, larger, and unshriveled ones, perhaps 

 immature, as shown by color or condition of septation. In species of Calicium 

 and other closely related genera the upper portion of the wall of the ascus becomes 

 gelatinized and dissolved before the spores are mature, and the spores escape and 

 ripen in the hymenium outside the asci. In other ascomycetous lichens the spores 

 mature within the asci, which then open at the apex for their escape. The apical 

 wall may rupture irregularly, the end may become torn across in some regular way, 

 or probably in many instances an apical plug is pushed out, as in some other 

 ascomycetous fungi. 



The Spores. These bodies are usually eight in each ascus and rather small 

 in size, but the number may vary from one to many and the size is inversely pro- 

 portional to number, varying from 3 //. to 200 (i or more in length. The most 

 common forms are oblong or ellipsoid, but the spores vary from acicular to spher- 

 ical. Usually the size and form are quite constant in a given species, but there 

 are instances of considerable variation even in the same hymenium. 



The spores as arranged in the ascus may be seriate, oblique, parallel, or more 

 or less irregularly placed. They are likely to be parallel when acicular in form, 

 uniseriate when spherical or oblong in a cylindrical ascus, and obliquely or irreg- 

 ularly arranged when the ascus is pyriform, thickly clavate, or ventricose-clavate. 



The spores may be non-septate, 1-septate, 3-septate, or several-septate, the 

 cells being, in most of the compound conditions, arranged in a single series, sepa- 

 rated by transverse walls. Besides the transverse divisions, others may be formed 

 in the direction of the long axis of the spore, giving what is known as a muriform 

 spore. In the development of the muriform spores the transverse septations 

 always appear first, and in some species only a portion of the spores are ever 



