MINUTE STRUCTURE OF LEAVES 55 



EXPERIMENT XXX 



How do young shoots of English ivy bend with lefeience to light ? Place a 

 thrifty potted plant of English ivy before a sipall window, e.g. an ordinary 

 cellar window, or in a large covered box, painted dull black within and 

 open only on the side toward a south window. After some days note the 

 position of the tips of the shoots. Explain the use to the plant of their 

 movements. 



Eepekence. Detmer-Moor, 9. 



41. Sun leaves and shade leaves.^ Select for study some species of shrub 

 or tree which furnishes a dense shade. Deciduous species will answer, but 

 broad-leafed evergreens, like hollies, some rhododendrons, or live oaks are 

 still better. Why ? Gather some of the outer leaves and some of the inner- 

 most ones from the same tree. Measure the per cent of total illumination 

 received by the innermost leaves, as described in Exp. XXVIII. Make a 

 detailed comparison of the two kinds of leaves (those grown in sun and in 

 shade) as follows : • 



A. Comparison of average areas (see Exp. XXXVIII). 



B. Comparison of hairiness or scaliness of the under surfaces. 



C. Comparison of thickness of leaves. Use a power of 25-50 diameters 

 and a micrometer eyepiece, if one is available. 



D. Comparison of details of structure of cross sections (see Sees. 42, 43). 

 Explain as fully as possible all the differences noted in comparisons 

 A-D above. 



Reference. Clements, 59. 



MINUTE STEUCTUEE OE LEAVES; FUNCTIONS 

 OF LEAVES 



42. Minute structure of lily leaf." * * 



A. The student should first examine with m.p. a cross section 

 of the leaf. This will show : 



1. The upper epidermis of the leaf, a thin, nearly transpar- 

 ent membrane. 



2. The intermediate tissues. 



3. The lower epidermis. 



1 A simpler study may be made by comparing the illuminations and structures 

 of characteristic sun plants and shade plants, for instance Portulaca, Sedum, etc., 

 with Arissma, Aralia, C'lintonia, Trillium, etc., each;^grown in its natural habitat. 



2 Any kind of lily will answer. Other leaves are equally good but many of 

 them are not obtainable at aU seasons. Some excellent kinds are Fuchsia, English 



