79 



of the thallus ; the lobes or branches are, however, very irregular in 

 their occurrence, true dichotomy not being recognizable. 



In the majority of lichens the algae are entirely enclosed by the 

 hyphal structure and their arrangement and development in the thal- 

 lus is directed and controlled by the hyphal symbiont. This pre- 

 dominance of the fungal portion is, however, more apparent than 

 real. We shall here consider only the arrangement and growth of 

 the algae in the thallus ; their structure and manner of division will 

 be discussed in the chapter on Algal Types ; their relation to the 

 haustoria has already been explained. 



In any given lichen the alga assumes its definite position as soon 

 as the thallus has reached its mature development. The area of 

 mature development begins near the apex, as has been explained. 

 The rate of multiplication and distribution of algae keeps pace with 

 the growth of the fungal portion. The position of the algae depends 

 largely upon the form of the thallus or other structures (cephalodia, 

 apothecia) in which they occur. To be more specific, it may be 

 stated that the algae take a position most suitable for the utiliza- 

 tion of sunlight and the exchange of gases resulting from the pro- 

 cesses of assimilation. In the horizontal thalli (crustaceous and 

 foliose) this is near the upper surface (bifacial thalli) ; in flattened 

 fruticose thalli the "algae occur in two layers, as both surfaces are 

 nearly equally exposed to the sunlight (centric thalli) ; in radial 

 fruticose thalli the algae are equally distributed on all sides. In the 

 thalloid exciple they occur in two circular layers, and this exciple is 

 therefore a combination of the centric and radial thalloid structure. 



The active division of algae takes place near the apical areas ; 

 in the typically horizontal thalli they never extend below the 

 apex ; from that point backward they increase in numbers until the 

 algal layer obtains its average thickness near the point where the 

 thallus reaches its normal thickness. New algal cells are formed 

 above and below a median horizontal plane through the algal layer. 

 The limitation of growth in thickness of the algal layer is determined 

 by several influences, first of all, by the requirements of assimila- 

 tion ; secondly, by the thickness of the superimposed structures of the 

 thallus. New cells are continually formed in the lower zone of the 

 algal layer, while the older ones in the upper zone die and the empty 

 cell-walls are carried upward by the growing hyphae and become in- 

 termingled with the protective coverings of the thallus. In general, 



