PHOTOSYNTHESIS AND RESPIRATION. 83 



minute but extremely numerous, and in bifacial leaves which 

 have distinct upper and lower surfaces they are usually more 

 abundant on the lower surface. 1 



The centre of the leaf is occupied by ground tissue, through 

 which strands of conducting tissue run, the latter being the 

 " veins " of the leaf. In bifacial leaves the ground tissue 

 (mesophyll, or central-leaf tissue) is usually divided into an 

 upper region in which its cells are long and narrow in the 

 vertical direction (i.e. lengthened at right angles to the 

 surface of the leaf), and a lower region in which the cells are 

 branched and loosely arranged. The upper tissue (palisade 

 tissue) is compact, the cells being close together or separated 

 only by narrow spaces, whilst the lower tissue (spongy tissue) 

 contains large spaces between the cells. The cells of the 

 palisade and spongy tissue all contain green grains; so do 

 the guard-cells of the stomates, but not the other cells of the 

 epidermis, nor those making up the veins. 



It is easy to discover that the spaces between the cells of 

 the middle tissue of the leaf contain air, and that they com- 

 municate with each other throughout the leaf, with the air- 

 spaces in the corresponding tissue (ground tissue, cortex) of 

 stem and root, and with the external air by means of the 

 stomates. The green grains (chloroplasts), present in the 

 cells of the middle tissue (mesophyll) and in the guard-cells 

 are spongy protoplasmic bodies containing liquid chloro- 

 phyll, which can be dissolved out of the grains, leaving them 

 colourless. 



* (a) The following is a good method of examining the structure of a 

 leaf. Boil some small entire leaves (those of Box or Privet answer 

 well) in caustic potash for about ten minutes. Hold a leaf under 

 water, and with scissors cut off a strip round the margin ; the leaf is 

 then readily separated, with the aid of a mounted needle, into three 

 parts, which should be mounted on separate slides and examined with 

 the lens, and then covered with a cover-glass and examined with the 

 microscope. The three parts are (1) the upper skin or epidermis, 

 (2) the mesophyll, containing the veins, (3) the lower skin. 2 



1 A square millimetre of Broad Bean leaflet gave, as the average 

 of several countings, 90 stomates in the upper epidermis, and 140 

 in the lower ; a leaflet with area 10 sq. cms. therefore bears over 

 200,000 stomates. 



2 See preparations of Box-leaf, made in this way, in the Plant 

 Biology Collection of Microscopic Slides. 



