LEAVES 81 



Glands. — These are epidermal structures which are extremely 

 variable. The simplest form is a simple trichome in which the 

 apical cell is large and thin-walled. A second type consists of a 

 group of cells in a more or less well-defined pit in the epidermis. 

 Glands of various kinds are found in other parts of the plant 

 (Fig. 62). So'me of the most important are the nectar glands in 

 flowers (Fig. 62) and the large glands in the peel of the orange. 

 The glands may secrete water or water containing sugars, en- 

 zymes or other products. The sticky character of some leaves 

 and stems and the odor of many plants come from the glands 

 (see Secretions, page 179). 



LABORATORY EXERCISES. 



Exercise 1. Examine a number of leaves of various plants to determine 

 the parts, forms, types, venation, arrangement and uses so far as possible. 



Exercise 2. Examine a cross-section of the leaf. Note the upper epi- 

 dermis, palisade cells, mesophyll cells, lo-wer epidermis, fibro-vascular 

 bundles, intercellular spaces, stomata, location of chlorophyll and thick- 

 ening of the outer walls of the epidermal cells. 



Exercise 3. Examine the epidermis from both the lower and upper sur- 

 faces of leaves of several plants and study the number and arrangement 

 of the stomata and the types of triehomes. Some of the grains should be 

 included in this series. 



Exercise 4- Examine the triehomes from the leaves and stems of a 

 number of plants, so as to get some idea as to the great variety. , 



