CHAPTER VII. PHENOMENA OF VEGETATION. LICHENS. 



403 



can at present only be described as not heteromerous, and under this head certain types 

 may be distinguished by positive characters, the most suitable of which are drawn 

 from the Algae which form part of each species, as will be explained below. 



i . The growth of the Fungus-body in the heteromerous thallus (Fig. 171) 

 is progressive in the direction of the apex in the fruticose Lichens, in the direction of 

 the margin in the others (see section XII). The apex or margin which takes the 

 lead in growth is formed of the terminal portions of the hyphal system, and these are 

 the form-elements which determine the course of the development. The Alga 

 follows in its growth the apex or margin which precedes it, moving after it, but always 

 continuing at a certain distance behind it. This growth by gradual advance may be 

 followed in successive transverse zones by intercalary surface-growth and growth in 



FIG. 172. Physcia parietina, Kbr. Section 

 through the young thallus; o upper rind-layer, 

 under rind-layer, g algal zone (CystotorcHs). 

 After Schwendener. Magn. 500 times. 



FIG. 173. Sticta fuliginosa . Transverse section through the 

 foliaceous thallus ; the letters as in Fig. 172, also m medullary layer, > 

 rhizoid strands. The algal zone here consists of a species of Chroo- 

 coccus with thick colourless gelatinous membrane and bluish green 

 protoplasm which is black in the figure. After Sachs. Magn. 500 times. 



thickness, caused by the formation and insertion of new hyphal branches and algal 

 cells and also by the increase in volume of those previously formed. 



The ramifications also in the foliaceous and fructicose thallus proceed from the 

 hyphal system. They are partly bifurcations, and partly arise ' adventitiously ' at 

 points not in most cases very exactly and morphologically defined. 



The structure also of the mature heteromerous thallus (Figs. 171-173) 

 is in its ground-plan that of the Fungus-body of the Lichen. We may as a 

 rule distinguish in it a comparatively thin (its average thickness is about 10 /*) 

 usually transparent dense peripheral rind-layer (stratum corticale) clothing the 

 free surface and a weft beneath the rind which is usually loose and everywhere 

 furnished with interstices containing air, the medulla or medullary layer (stratum 



D d 2 



