INTRODUCTION 3 



the flat or horizontal ones may be either foliose or crustose : f oliose when somewhat 

 leaflike and crustose when forming a closely attached crust resting on or within 

 the substratum. Various intermediate conditions between crustose and foliose and 

 between foliose and fruticose forms may be expected. 



General Forms of Thalli. Of the foliose forms, many are variously lobed, 

 and some are almost entire at the margin. In instances where the lobing is evident, 

 the lobes may be variously imbricated. In both lobed and unlobed forms the 

 margin may be wavy or crenate instead of entire, and it may be either ciliate or 

 devoid of cilia. The most obtrusive differences among the fruticose thalli consist 

 in their being branched or unbranched, and, when branched, in the manner of their 

 branching. Secondary differences are found in the character of the surface, par- 

 ticularly in various small outgrowths other than the branches, known as phyllocladia. 

 The crustose thalli appear usually as a more or less conspicuous layer spread over 

 the substratum or sometimes really lying wholly or partly in it and indicated at the 

 surface often only by a change in color. These crustose thalli will be found to be 

 irregular in outline or more or less plainly orbicular and to form a continuous or 

 a variously broken and scattered stratum. In some species the tendency is 

 toward orbicular forms and in others toward irregularity in form, but in any case 

 the peculiarities of the surface of bark, old wood, or rock forming the substratum 

 will determine the form of the particular thallus to a large extent. 



Sizes of Thalli. In the northern and northwestern states the fruticose 

 thalli of Usnea longissima, which grow in tangled masses hanging over the branches 

 of trees, frequently reach 1.5 meters in length, while the foliose thalli of Gyrophora 

 Dillenii sometimes reach 35 cm. or more in their largest diameter. Both fruticose 

 and foliose thalli may vary from these large forms to minute ones not more than 

 0.2 mm. in height or diameter. In the crustose thalli the spread over and within 

 the substratum may vary greatly, but is seldom more than 10 cm. In these and 

 the foliose forms the thickness is to be taken into account. In the descriptions, 

 however, actual measurements are very seldom given, though comparative state- 

 ments are often employed. 



Surfaces of Thalli. In the foliose thalli the upper surface is comparatively 

 smooth, wrinkled, corrugated, or pustulate, and it may bear cilia, soredia, or the 

 minute growths known as isidioid or coralloid branchlets. The margin of the 

 thallus may be closely attached to the substratum, or more or less ascending. The 

 lower surface is variously covered with the attaching organs known as rhizoids. 

 These may be large or small, numerous or few, and evenly scattered or collected 

 into rows or into groups of different forms. The lower surface is sometimes quite 

 smooth except for these rhizoids, but may be variously wrinkled or pitted, or, in 

 one species of Gyrophora, provided with vertical plates which give strength. 



In the fruticose thalli the surface is either smooth or variously pitted, 

 and in some instances it is somewhat tomentose. The species of Cladonia put 

 forth as superficial outgrowths the flat expansions known as squamules. The 

 species of Stereocaulon bear the peculiar structures more irregular in form called 

 phyllocladia. The form, size, frequency of occurrence, and distribution of these 

 organs must be noted carefully. In species of Cladonia, especially, it is necessary 

 to observe whether the cortex of the podetium is entire or somewhat broken, 

 so that it becomes areolate or even disappears over some portion of the organ. 



Finally, turning to the crustose thalli, they are also smooth or variously 

 roughened. Those that are hypophloeodal or hypolithic simply take the contour 

 of the surface of the substratum, as do also some thin and smooth forms that 



