Cavers: Some Pot'n/s in the Biology of Ifepa/iece. 17, 



llie thallus bears loni;-, colourless root-hairs and a series of thin 

 i^reen scales ; each of the latter becomes after a time split in the 

 middle, but the two lateral rows of scales thus formed ultimately 

 become withered in the older parts of the thallus. In the species 

 of Ri'ccia which grow in drier and more exposed situations, e.g., 

 R. nigrella, the scales, which are large and violet-coloured, 

 appear to play an important part in enabling the plant to resist 

 tlie effects of extremes of temperature or of prolonged periods of 

 drought. Under such adverse conditions the plant becomes 

 rolled up, with the 



ventral surface out- -rnrT'TT~rrTXT7~rTT7cn: 



wards, covered by the 

 closely -overlapping 

 scales. In other 

 species with a similar 

 xerophytic habitat, 

 e.g., R. cilia ta, R. 

 tumida, the epidermal 

 cells on the upper sur- 

 face, especially at the 

 margins of the thallus, 

 grow out as long, 

 colourless hairs, which 

 doubtless serve to re- 

 lain water. In R. oys- 

 tiilliiia, which grows 

 on wet stones i n 

 streams, the air-spaces 

 become drawn out into 

 wide chambers which remain open above, forming a series of 

 shallow cavities separated by thin vertical partitions. These 

 partitions form a network, as seen from above, and give this 

 species its remarkably crystalline appearance in the fresh and 

 moist condition ; the ventral scales are very small, each consist- 

 ing of a short row of cells and soon becoming withered off. In 

 .Ricciella flu i tans the air-spaces grow in width, but at the same 

 time the epidermal cells grow out horizontally so as to form 

 a roof over each chamber. If the plant is growing on the soil 

 the roofing-in of the chambers remains incomplete, a small pore 

 being left above each chamber, but in the sterile aquatic plants, 

 which grow submerged in water, the chambers are completelv 

 closed. In Ricciocarpus iiatans the broad heart-shaped thallus 

 is thick and spongy, consisting almost entirely of air-chambers 



1905 May I. 



Fig. i. — Ricciocarpus natans. I., Part of a traiis- 

 \erse section of the thallus ; II., epidermis in surface 

 view, showing- two pores; III., terminal portion o( 

 one of the long pendant scales, x 30. 



