STUDIES OF rUE LEAF 69 



interest to determine the effect of low temperature on the sensitiveness 

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

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

 humidity of the surrounding air may be ascertained by keeping some 

 weli-nioistened young plants under a bell jar, and comparing with 

 others kept in a very dry place. 



Exercise XXIV 



(1) Tlie parls of a ti/pical leaf. — Draw the given leaf in simple 

 outline to show the hlude; the petiole, nr stalk; tjie stipules (a pair of 

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



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

 by transmitted light, shielding it from direct light. 



Note : — (a) The translucence. 



(5) The distribution of the green color, 

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

 l)lade (use direct light). 



Trace the main fi-amewoik of one half of the leaf, including in the 

 drawing only the most prominent rilis ami 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 bi-aiiches fi'oni this, — and so on; 

 counting the numlier of turns made to arrive at the smallest veiiilefs 

 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 watei- colored witli 

 eosin, and later tr.ace 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 ? ^ 



Exercise XXV 



Take a shoot of the Pea three or four weeks old at leai?t, with several 

 leaves fully formed and a growing bud. 



Note the stipules. Where is ihe 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 np readily 

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

 have the petioles raised above water. 



