STUDIES OF THE LEAF 69 



interest to determine the eifect of low temperature on the sensitiveness 

 of the plant; temperature between 40° and 50°, for instance, to which 

 the plant has been exposed for a few hours. The effect of varying the 

 humidity of the surrounding air may be ascertained by keepino- some 

 well-moistened young plants under a bell jar, and comparing with 

 others kept in a very dry place. 



Exercise XXIV 



(1) The parts of a typical leaf. — Draw the given leaf in simple 

 outline to show the blade; the petiole, or stalk; the stipules (a pair of 

 members at the base of the petiole, like leaflets). 



(2) The structure of the blade ^ — Examine the blade under the lens 

 by transmitted light, shielding it from direct light. 



Note : — (a) The translucence. 



(Jj) The distribution of the green color, 

 (c) The relative thickness of the ribs and the rest of the 

 blade (use direct light). 



Trace the main framework of one half of the leaf, including in the 

 drawing only the most prominent ribs and their conspicuous connect- 

 ing veins. 



How many ranks or orders of ribs and veins do you distinguish? 

 Determine this as follows : Follow the midrib, then one of its large 

 branches, then one of the main branches from this, — and so on; 

 counting the number of turns made to arrive at the smallest veinlet's 

 end. 



Draw a small square to show the veinlets of the two or three lowest 

 ranks, as seen through the lens. 



Experiment 20. — Place a leaf with its stalk in water colored with 

 eosin, and later trace the water courses of the leaf. 



Experiment 21. — Take a wilted leaf, and after noting with care how 

 flaccid it is, put it entirely under water for a day. Then note again 

 the degree of rigidity. 



Does contained water play any part in the support and stability of 

 the leaf blade? 2 



Exercise XXV 



Take a shoot of the Pea three or four weeks old at least, with several 

 leaves fully formed and a growing bud. 



Note the stipules. Where is the growing tip of the shoot, and how 

 is it protected? What two uses do the stipules here subserve? The 



1 For the minute structure see Chapter XVII. 



2 To determine whether in this experiment water is taken up readily 

 through the general surface, use several uninjured leaves, some of which 

 have the petioles raised above water 



