106 DR. K. LEDEGANCK ON THE FALL OF LEAVES IN AUTUMN. 



the bougli will give information as to tlie modification of tlie peri- 

 dermal tissue. 



The re-agents most generally used are : — 



1. — Tincture of iodine and sulphuric acid for the detection of 

 cellulose, by the fine blue colour which they give it. 



2. — Schulz' test for the same purpose. 



3. — Solution of copper in ammonia, which dissolves cellulose, 

 and separates corky, cuticular, and woody tissues. 



4. — Aqueous solution of iodine, which discloses the presence of 

 starch in the cells, and gives a brown colour to suberous, peridermic 

 and cuticular tissues. 



5. — Alcohol and ether, to dissolve fats and resinous substances. 



The following is a short account of our mode of procedure with 

 each species : — 



We examine in succession the leaves from a green bough, those 

 from a withering one (from which they have begun to fall) ; and 

 lastly, the scar left upon the stem after the fall of the leaf. Both 

 the green and yellow leaf are examined in the state of articulation, 

 with the pulvinus, by longitudinal sections, either radial or 

 tangential across the pulvinus and the base of the petiole. We 

 notice accurately the state of these parts in a fresh state, before 

 proceeding to make use of any test. Transverse sections of the 

 petiole and stem, when thought necessary, give a true idea of 

 the disposition of parts about the articulation. Finally, the use of 

 reagents discovers the existence of such and such tissues, according 

 to the time at which examination is made. The brittleness of the 

 articulation in autumn, of leaves about to fall especially, sometimes 

 offers insuperable difficulties. In such case, the contiguous 

 surfaces must be examined separately, as soon after separation as 

 possible. 



///. — General Considerations. 



In its normal state, the insertion of the leaf upon its stem is either 

 by articulation or by continuity. In the first case, a special organ, 

 the pulvinus, acts as an intermediate organ between the stem and the 

 leaf; in the second, a simple, annular constriction at the base of 

 the petiole points out the anatomical point where the leaf begins. 

 The pulvinus, a little lateral swelling of the stem, presents the 

 same anatomical elements as the cortical part, only disposed in a 

 slightly different manner ; the medullary tissue of the stalk, in- 



