124 Practical Plant Biology. 



obstructions and the inside of the wall of the pore are coated with 

 a waxy material which is not easily wetted, so that a drop of water 

 finds it very difficult to enter by the pore. Immediately below 

 the superficial layer are the air-chambers of the thallus. They 

 are separated from one another by small vertical walls one or two 

 cells thick resting on the floor of these chambers. The cells 

 forming these walls and the floors of the chambers are similar to 

 those of the superficial layer in size and form, and like them they 

 contain chloroplasts. The cavities of the chambers are partially 

 filled with dark green cells. Examination of these shows that the 

 cells are somewhat barrel-shaped and they are grouped together 

 in short filaments 3 or 4 cells in length. These filaments grow 

 up from the floor of the chamber and extend into its cavity, being 

 bathed in the air filling the latter. Sometimes they are branched. 

 The cells forming them have very thin walls, and in their proto- 

 plasm there are great numbers of chloroplasts so that the filaments 

 have an intense green colour. Evidently this whole green layer 

 of the thallus is a highly specialised arrangement whereby these 

 cells with their photosynthetic apparatus, covered only with 

 delicate walls, may be protected from desiccation, even though not 

 submerged in water. The smallness of the pores leading into the 

 chambers secures that the air currents will not carry off too rapidly 

 the water vapour surrounding the cells, while danger of blocking 

 with water and thus cutting off the supply of carbon dioxide is 

 minimised by the waxy lining and the obstructing processes of the 

 pore. 



Beneath the layer of small cells containing chlorophyll which 

 form the floor are several layers of large cells with little or no 

 chlorophyll. These cells often contain a certain amount of starch 

 and always a large watery vacuole. Their function seems to be 

 to hold a supply of water for the green cells and to store the 

 material these have formed in photosynthesis. Their walls are 

 relatively thick, sometimes they have narrow groove-like marks 

 upon them which are thin places on the walls ; these are called 

 pits. Probably the latter facilitate the movement of liquid from 

 one cell to another. 



The lower surface of the thallus is formed of two or three 

 layers of small irregular cells. Their walls are often coloured 

 violet or brown, they are without chlorophyll. The rhizoids are 

 seen to be outgrowths from some of these cells at the surface. 

 They are of two kinds ; rhizoids with straight tubular walls, 

 smooth on the inside as well as the outside, and others with 

 constricted walls furnished on their inside with numerous peg- 



