212 



Cavers: Notes on Yorkshire Bryophytes. 



RehouUa. When once the oil-bodies are formed, they appear to 

 remain unchanged until the death of the cells containing them ; 

 plants may be kept in darkness for weeks, and the new parts 

 formed are found invariably to contain oil-bodies, whilst those 

 already present in the older parts remain unaltered. These 

 bodies can therefore only be regarded as products of excretion, 

 but they appear to play an important part in the economy of the 

 plant, since, as shown by Stahl'"", they serve to protect the thallus 

 against the attacks of snails and other animals. Snails or slugs 

 will shun fresh pieces of the thallus even when there is no other 

 available food, but if the pieces are soaked in alcohol (to remove 

 the oil) and washed in water, they will be readily eaten by these 

 animals. 



Other cells of this compact tissue contain mucilage, which 

 readily absorbs and stores up water. Both oil and mucilage 

 cells are sometimes found in the partitions between the air- 

 chambers, but they are most abundant in the compact tissue of 

 the midrib. The lowest cells of this tissue, especially in the 

 thin lateral parts of the thallus, frequently have their cell-walls 

 coloured deep red or purple. 



On its lower surface the midrib bears two rows of scales, one 

 on either side of the middle line, together with numerous rhizoids, 

 the latter being of two distinct kinds, described below. 



The VENTRAL SCALES arise in two rows from the sides of the 

 midrib. They are broad and closely overlap each other, but do 

 not reach the margins of the thallus. Each scale is roughly 

 semicircular or scimitar-like in outline, with a narrow basal 

 prolongation which extends inwards nearly to the middle line 

 of the thallus, while its free roanded margin bears in most cases 

 two long, narrow, pointed appendages (Fig. 2, B.). Frequently 

 the scale bears only one of these appendages, occasionally there 

 are three of them. In the youngest scales, near the growing- 

 point of the thallus, these appendages curve upwards and 

 occupy the groove in which lies the row of initial-cells. The 

 appendages are developed before the rest of the scale, and have 

 become deeply tinged with purple while the broad basal part of 

 the scale is still green or colourless. The cells of the young 

 scale contain protoplasm, chloroplasts, and starch-grains, and 

 the marginal cells frequently grow out as club-shaped mucilage- 

 hairs. The fully-developed scale consists of a single layer of 

 cells, most of which have lost their original contents and have 

 deeply-coloured walls. At the base of the scale, along its 



* Pflanzen und Schnecken, Jena, 1888 ; p. 49. 



Naturalist, 



