HISTOLOGY OF ANGIOSPERM STEM, BOOT, ETC. 125 



Note that floating leaves have the stomata on the upper 

 side, and very large air-spaces in both the palisade and 

 spongy mesophyll often forming wide air-chambers below 

 the stomata ; while submerged leaves have chloroplasts in 

 the ordinary epidermal cells, no stomata, feebly developed 

 vascular bundles, and small air-spaces. 



158. Hairs, Glands, etc. The leaves (and also the 

 stems) of various plants should be examined for different 

 types of hairs and glands. In each case strip off the 

 epidermis (or make tangential sections) and also cut 

 transverse sections of the leaf or stem. 



The following show interesting hairs : Wallflower (hairs com- 

 pass-shaped) ; Shepherd's Purse (hairs star-shaped) ; Stinging Nettle 

 (large unicellular stinging hairs, sunk in a multicellular base and 

 provided at tip with a detachable swelling) ; Goosefoot or Orache 

 (large spherical or ovoid shortly stalked hairs, giving the leaves a 

 mealy appearance) ; Hop (compass-like hairs with stalk sunk in a 

 raised multicellular basal outgrowth these hairs help the plant in 

 climbing) ; Goosegrass (curved and pointed hairs, which help the 

 plant to climb) ; Mouee-ear Hawkweed (shaggy hairs, consisting of 

 several longitudinal rows of cells cohering laterally). 



Various forms of glandular hairs should also be examined, e.g. 

 those on petiole of Chinese Primrose, leaves of various Labiates, 

 etc., which have a multicellular stalk and a rounded glandular 

 terminal cell ; the short thick rounded multicellular glands on the 

 bud-scales of Horse Chestnut. In these and various other cases, 

 the copious secretion of the gland may be seen in sections mounted 

 in water. This secretion may be resinous or oily. Of special 

 interest are the gland hairs found on the leaves of Sundew and 

 Butterwort, which produce enzymes for the digestion of insects 

 caught by the sticky secretion. 



159. Structure of Perianth-leaves of Flower. 



Examine the perianth-leaves of various Monocotyledons, 

 and the sepals and petals of various Dicotyledons. In 

 some cases these leaves are so transparent that they may 

 be mounted entire, or made transparent by treatment with 

 chloral hydrate, potash, etc. In other cases, make tangen- 

 tial and transverse sections as in the case of foliage- leaves. 

 Note that the perianth-leaves of Tulip, etc., have stomata 

 in the epidermis ; this is often the case also with the 

 sepals and even the petals of various Dicotyledons, but the 



