In the higher crustaceous lichens and in all foliose forms, marginal 

 growth is no longer uniform ; certain parts show a more marked 

 proliferation and are recognized as apical areas. It is evident that 

 at these points the curves of growth are radial instead of in a 

 single plane (vertical); the lateral curves are usually much the 

 longest, and they do not terminate at right angles to the median 

 plane. The rate and activity of apical growth determines the size 

 and form of the thallus-lobes or of the thallus. Sometimes the upper 

 hyphae curve more than the lower or vice versa, which causes the 

 apical portion of the thallus to curve either upward or downward. 



The differentiation of tissue elements begins very early in the de- 

 velopment of the thallus. At a very short distance from the apex 

 the tissues are fully formed and the thallus has reached its average 

 thickness. On an examination of the apical area it becomes evident 

 that the new cells are cut off at a short distance behind the apex, 

 since the changes which they present are of secondary origin ; they 

 are shorter and their walls thicker than the normal hyphal cells, but 

 longer and less differentiated than the cortical. 



All gradations between the mode of growth of the lowest crusta- 

 ceous lichens and that of the most perfect foliose forms can readily 

 be found ; to enter into a discussion of these multitudinous forms of 

 growth is here unnecessary, as the descriptions of families and gen- 

 era will do much to clear up points of detail. 



Vertical Growth. 



As already indicated, fruticose thalli present either a centric or 

 radial structure, and accordingly the mode of growth somewhat 

 differs. Leaving out of consideration the question of direction, the 

 development of fruticose thalli proceeds much as in horizontal thalli. 

 The following are some of the differences : The apical areas are 

 more clearly marked and more definite in their occurrence ; branch- 

 ing is typically dichotomous, which, however, is also well marked in 

 the higher foliose thalli, but indistinct or wanting in the lower foliose 

 and crustaceous lichens ; the apical areas are also less blunt, and 

 the lobes or branches do not obtain their full thickness as quickly as 

 they do in typically foliose lichens ; the number of branches indicates 

 the number of apical areas which had formed. No explanation has 

 as yet been given of the main stem and branches taking their definite 

 position. They are, no doubt, controlled by the same laws that con- 



