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. 
1. 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 
ee 
oA NOR 
eee 


F1G.172. Physcia partetina, Kbr. Section FIG. 173. Sticta_fuligzs T section the 
through the young thallus; o upper rind-layer, foliaceous thallus ; the letters as: in Fig. 172, also m medullary layer, r 
% under rind-layer, g algal zone (Cystococcus). rhizoid strands. The algal zone here consists of a species of Chroo- 
After Schwendener. Magn. 500 times, coccus with thick col b and bluish green 

protoplasm which is black in the figure. After Sachs, Magn. soo 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 p) 
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 
pd2 
