372 A TEXTBOOK OF THEORETICAL BOTANY 



root-like organs termed rhizinae. The inner cores of these consist of loosely 

 packed, longitudinal hvphae, which are surrounded by a loose tissue con- 

 tinuous with the hypothallus. These rhizinae are employed partly for 

 attachment, but mainly to absorb water, which they do chiefly by means of 

 the anastomosing hvphae at their apices. Water is passed from them through 

 the veins to the gonidial layer where it is used up during photosynthesis. 



Router 



CORTEX 



INNER 

 CORTEX 



GONIDIA 

 LAYER 



CARPO- 

 GONIUM 



MEDULLA 



PARAPHYSES 



ASCOGENOUS 

 HYPHAE 



ASCOGONIAL 

 CELLS 



Pic ^.^_ — A, Phvscia piilvendenta. Coiled carpogonium and trichogyne before 

 fertilization. B, Physcia ciliaris. Ascogonia with developing ascogenous hyphae, 

 (A, After Darbishire. B, After Baiir.) 



Our knowledge of the interchange of food between the Alga and the Fungus 

 is still very incomplete. 



Peltigera possesses soredia and isidia. The soredia are small and repro- 

 duce the whole Lichen. The isidia develop as outgrowths of the Lichen 

 thallus ; they are developed where conditions are moist and probably have 

 some physiological function. It has been suggested that they assist in 

 increasing Carbon assimilation. One species, P. ophthosa, possesses remark- 

 ably large, dark-coloured cephalodia, which are scattered over the surface 

 of the thallus. 



Sexual Reproduction 



The apothecia arise from carpogonia, which are developed in the 

 medulla of the young lobes of the thallus. Each consists of a coiled multi- 

 cellular filament of large cells, terminated by a multicellular trichogyne, 

 which projects beyond the upper surface of the thallus. The cells of the 

 trichogyne are elongated and have pit connections between them. A number 

 of carpogonia form at each point where an apothecium is developing, though 

 possibly only one becomes fertile. The coiled structure, the absence of 

 auxiliary cells and the multicellular trichogyne are all points which distinguish 

 the Lichen carpogonial system from the somewhat similar structure in the 

 Rhodophyceae (Fig. 358). 



