74 



trol the direction of growth in higher plants. In a number of in- 

 stances the secondary branches assume a position nearly horizontal, 

 as in Usnea barbata and the apothecial branches of some Cla- 

 donias, but they are more frequently directed diagonally upward. 



In this form of lichen growth the fungal symbiont likewise ini- 

 tiates and controls the mode of development ; there are, however, 

 notable exceptions, as we shall learn in the discussion of algal growth 

 and development. It must also be borne in mind that in some lichens 

 there is a combination of horizontal and vertical growth, as, for ex- 

 ample, in Cladonia ; this is more specifically an association of a foli- 

 ose and a fruticose development. In Cladonia the so-called primary 

 thallus is flattened ; it develops in the manner of a typical foliose 

 thallus, but the podetium is typically fruticose in structure and mode 

 of growth. This peculiar difference of growth is readily explain- 

 able when we consider the phylogenetic development of the pode- 

 tium ; morphologically and physiologically this organ is a thallus, 

 but originally it was no doubt an apothecial structure which dur- 

 ing its phylogenetic evolution became converted into a thallus ; it 

 takes its origin endogenously from the primary thallus in the manner 

 of apothecia proper ; it grows from a single apical area and upon 

 pushing its way through the upper layers of the primary thallus de- 

 velops into a podetium with a typical radial structure. The podetium 

 is of interest from a biological standpoint, since it gives evidence of 

 the constancy of morphological characters. Although the present 

 functional activity of the podetium is entirely thalloid, it has retained 

 many of the morphological characters of the apothecium. For a 

 more complete discussion of this subject see the excellent paper by 

 Reinke (75, I). 



(c) Intercalary Growth. 



In intercalary growth new cells are interpolated and existing cells 

 are increased in length and width. In general, intercalary growth 

 causes the thallus to increase somewhat in surface expansion and in 

 thickness. The interpolation of new cells is most active near the 

 margin of the thallus, near the peripheral portions, and in the algal 

 zone. The internal tissues, medullary hyphae and central con- 

 ducting tissues, which are also mechanical are modified by the 

 elongation of their cells. In the cortical layers and dermis of the 

 foliose thalli the cells become shortened and their walls thickened ; 



