INTRODUCTION 13 



stated, the algal cells simply increase in number as the thallus increases in size. 

 Thus the algae always sustain a physiological relation to the growth of the thallus 

 and perform the function of assimilation. 



The Ascocarp in General. The fruit of an ascolichen is commonly called an 

 ascocarp, and consists of an epithecium, a hymenium, a hypothecium, and a thal- 

 loid or a proper exciple or both. Either or both of the exciples, however, may 

 disappear, or the proper exciple may be produced into a structure known as a 

 perithecial wall. Also, when a perithecial wall is present, it sometimes incloses an 

 additional structure known as an amphithecium. These structures may now be 

 explained in order. 



The Epithecium. This structure is supposed to be a film of the thallus ex- 

 tending over the upper surface of the ascocarp, and its presence is explained by 

 the fact that the development of the ascocarp begins within the thallus and that 

 the overlying part of this is carried up with the ascocarp as it finally bursts 

 through the upper cortex of the thallus. The epithecium is of the same color as 

 the upper portion of the hymenium and is usually, when present, not distinguishable 

 from it. Indeed, many of the older lichenologists considered this portion of the 

 hymenium a part of the epithecium and spoke of the epithecium as having certain 

 colors, when the color was in the upper portion of the hymenium. In the descrip- 

 tions to follow the epithecium has been ignored as something too rudimentary to 

 be distinguished in an ordinary examination of a fruit, or, as probably in the large 

 majority of lichens, entirely absent in mature ascocarps. Special statements are 

 made, however, of the color of the upper portion of the hymenium when differing 

 from that of the lower portion. 



The Hymenium. The hymenium is composed of the asci or spore-containing 

 sacs, and the protective filaments called paraphyses. In position this structure lies 

 below the epithecium and above the hypothecium. The asci are always thicker and 

 usually shorter than the paraphyses, and the mature spores may usually be dis- 

 tinguished in them more or less plainly. It need be further stated here only that 

 the asci and paraphyses are usually erect or suberect and constitute a densely 

 packed mass composed of the two structures. 



The Thalloid Exciple. This structure is commonly found in lichens having 

 a well-developed thallus, whether fruticose or foliose, and is found, therefore, most 

 commonly in the foliose forms and least commonly in the crustose ones. In 

 structure the thalloid exciple resembles the thallus, with which it is always con- 

 tinuous. The outer layer of such an exciple is directly continuous with the upper 

 layer of the thallus, whether a true plectenchymatous cortex or a pseudocortex. 

 This cortex of the exciple is often thicker than that of the thallus, to serve both 

 for protection and support to the structures within and to aid in the dispersal of 

 the spores. Within the cortex of the thalloid exciple there may be an algal layer 

 and within this a medullary layer, but these two layers are by no means always 

 clearly differentiated. There is no layer in any thalloid exciple that corresponds 

 to the lower cortex found in so many foliose lichens. Occasionally the algae may 

 disappear from the thalloid exciple with age, but such a structure is still a thalloid 

 exciple. The structure of the thalloid exciple is perfectly plain if the ascocarps are 

 adnate, sessile, or pedicellate, but if they are immersed it is by no means so easy 

 to determine whether the portion of thallus surrounding should be regarded as a 

 thalloid exciple. Also, if the thallus itself is of a rudimentary character and 

 devoid of distinct differentiation into layers, the thalloid exciple may be present 

 in a similarly rudimentary condition, often as a thin veil, which may be evanescent. 



