12 LICHEN FLORA OF THE UNITED STATES 



saucer-shaped depression, whose bottom and sides are covered with a dense coating 

 of short hyphal branches. The position of the cyphellae over the breathing pores 

 would seem to indicate that they serve to admit air to the interior of the thallus. 



Breathing Pores. In lichens having a thallus well developed on both sides, 

 it is evident that some provision for the admission of air to the interior will be 

 advantageous. This is especially true where the plectenchymatous cortex is quite 

 thick. The breathing pores of the lower cortex in species of Sticta have just been 

 mentioned. Such structures are found in the lower cortex of many other lichens, 

 but are even more common in the upper plectenchymatous cortex. They consist of 

 more or less branched pores extending from the surface of the algal layer upward 

 through the cortex in a somewhat irregular course and having no proper wall of 

 their own but forming simply intracellular canals. They resemble stomata in that 

 they can be closed, and, also, as is true of stomata, it is doubtful whether they are 

 really of greater use for the exchange of gases than for some other purposes. They 

 are found closed when the thallus becomes dry, and the closing doubtless helps to 

 prevent the transpiration of moisture. Similar openings between the hyphae of 

 pseudocortices may sometimes be made out, especially when these cortices are 

 thick. However, whether the cortex is plectenchymatous or not, these pores are 

 to be looked for, for the most part, in the thinnest portions of the cortex, espe- 

 cially over areas where the algal cells are numerous. 



Growth of Thalli. In crustose and foliose lichens the growth is for the 

 most part horizontal and mainly at or just behind the margin of the thallus. The 

 hyphae in all portions of the thallus, however, may increase in length, either by 

 formation of new cells or by the elongation of old ones. In some instances the 

 initial cells are equally active along the whole margin and the thallus is not lobed. 

 But in some of the higher crustose forms and the majority of the foliose forms 

 there are certain marginal areas where the initial cells are especially active, so that 

 the horizontal growth is more active here than elsewhere. This uneven growth 

 along the margin gives rise to the lobing so common in foliose lichens. Doubtless 

 in the lower crustose forms, where lobing is so uniformly absent, the frequent 

 irregularity in form is caused partly by the irregularities in the surface of the 

 substratum which impede growth more at certain points than elsewhere, and partly 

 also by the loss of portions of the epiphloeodal or epilithic thallus by abrasion. 

 In all instances of lobing the form of the lobes is determined by the size and 

 activity of the areas of initial cells whose division forms the lobes. The thallus 

 reaches its full thickness a short distance back of the growing margin. There is 

 no further increase owing to the fact that, while new cells, both algal and fungal, 

 are formed internally, abrasion of the general surface disposes of dead superficial 

 cells to the same amount. 



In most fruticose thalli the growth is mainly vertical instead of horizontal. 

 There may be a single apical region, consisting of a continuous area of initial cells, 

 and in such instances the thallus or the podetium will be unbranched, but generally 

 there are areas of special activity, one corresponding to each branch of the thallus, 

 whether horizontal, vertical, or lying at some intermediate angle. Here, as in all 

 thalli, the manner of branching and the number of branches will depend upon the 

 number and disposition of these areas of special activity. 



Sometimes the growth of the algal colony determines the size and form of the 

 so-called thallus. This is so in Ephebe pubescens, in which the dichotomously 

 branched alga appears to be the thallus, but the real thallus consists of hyphae within 

 the sheath of the alga. In the species of the Collemaceae, the hyphae have penetrated 

 more or less throughout the algal colony, thus giving the appearance of a some- 

 what foliose thallus. In the great majority of lichens, however, as already 



