8 THE BROAD BEAN PLANT. 



2O. The Leaflet. Notice the chief vein (midrib), which runs 

 through the leaflet from the base to the tip. What do you notice 

 at the tip? Note the branch-veins arising on each side from the 

 midrib, the finer veins arising from these, and the delicate net-like 

 arrangement of the still finer veins (hold the leaflet against the 

 light). What differences do you notice between the upper aud 

 lower surfaces of the leaflet? Notice that the chief veins appear as 

 grooves on one surface (which ?) and as projections on the other. 



Hold the leaflet, with the lower surface towards you, between the 

 thumb and forefinger of each hand, and, starting from a point on 

 one edge, tear the leaflet across. Notice the thin colourless skin which 

 can be torn off in this way, and the green inner tissue which is 

 exposed ; small specks of the green tissue remain attached to the 

 skin, unless the tearing is done very carefully (try it several times, 

 starting at different points on the edge of the leaflet). Now try to 

 tear off the upper skin in the same way, and notice that it also is 

 colourless, though a good deal of the green tissue always comes 

 away with it. The leaflet therefore consists of a thin colourless skin 

 covering its upper and lower surfaces, and a thick middle green tissue 

 which is denser towards the upper surface and looser towards the lower 

 surface (how is this inferred from your tearing experiments ?) and which 

 contains the veins. 



21. "Bloom" on Leaf-surface. Dip a leaflet into water and 

 notice, on lifting it out, that the water runs off the surface very 

 quickly, leaving it dry. Lay a leaflet on the table and place a drop of 

 water on either surface in turn : notice in both cases that the water 

 forms a globule instead of spreading over the surface. This shows that 

 the leaf is not easily wetted by water, and suggests that the surface is 

 covered by some substance like wax. This waxy substance is similar 

 to that which forms the " bloom" on Grapes, Plums, etc., so that one 

 might say the Broad Bean leaflet is covered with "bloom," which 

 prevents water from wetting it and reaching the inner tissue. Try to 

 remove the bloom by rubbing the leaflet gently (1) with a dry cloth, 

 (2) with a cloth dipped into warm water ; water placed on the leaflet 

 will then wet the surface and spread over it. 



22. The Stipules. How does a stipule differ from a leaflet in 

 shape, colour, and texture? Has it any veins (hold it against the 

 light) ? Notice the patch near the centre of the stipule : at first it 

 is bright red or violet in colour, later it becomes brown or black. 

 On which surface of the stipule is this patch most easily seen? 

 This patch consists of minute glandular hairs, very closely crowded 

 together on the lower surface of the stipule. A similar patch of 

 glands occurs on the stipule in the Common Vetch of fields ( Vicia 

 sativa), the nearest ally of the Broad Bean among British plants ; 

 its functions are doubtful. 



