234 XVIII. STRUCTURE OP THE BRYOPHYTA. 



simple microscope or a very low power, we can determine, by the 

 aid of needles, the existence of scales (amphigastria) which arise 

 from the surface of the thallus. Three different forms of ventral 

 scales are present : marginal, which usually extend somewhat 

 over the edge of the thallus, and have become brown ; median, 

 which lie in the middle line ; and laminar, which are inserted upon 

 the thallus on both sides of the middle line, but can also be want- 

 ing. The median scales, often purple coloured, alternate with one 

 another, and their edges overlap on the midrib. So far as the 

 median or laminar scales, or the former only, extend, arise from 

 the frond fine rhizoids, which, covered by the scales, and following 

 their insertion, extend to the midrib, and there run farther forwards 

 in bundles. It is the median and laminar scales which produce 

 the striation on the under side of the thallus, which we have 

 already observed with the naked eye. 



If we examine the dorsal (upper) side of the thallus with the 

 lens, this appears to be divided into small diamond-shaped areas. 

 The limits of the areas are dark green, the areas themselves 

 appear more grey. In the middle of each area a dot-like opening 

 is visible. 



We now examine, with stronger magnification, a surface- 

 section which is taken parallel to the dorsal side of the thallus. 

 We see that the outer cells of the dorsal surface are polygonal, 

 closely united together, and contain numerous large chlorophyll 

 granules. The boundaries of the areas show clearly ; each area 

 has its centre occupied by a round opening, which is surrounded 

 usually by four narrow crescent-formed cells containing no chloro- 

 phyll (Fig. 93, A}. Where the section is somewhat thicker, air 

 is seen to be collected under the free outer surface of the area. 

 Into this air space, the air-chamber, project chlorophyll-con- 

 taining threads of cells. The walls bounding the air-chamber 

 laterally are formed of closely- joined cells. These walls are of 

 one to several layers ; their cells contain chlorophyll. Individual 

 cells of the surface, and also of the interior, are distinguished by 

 a highly- refractive, irregularly-outlined, clustered body. These 

 bodies in the younger shoots are slightly brownish, in older are 

 coloured brown, contain usually only fat oil, and form the so- 

 called oil-bodies of the Liverworts. The cells which contain such 

 a body show no other plastic contents. 



Surf ace- sect ions taken from the central side of the thallus 

 show no division into areas. The cells are here more elongated 



