54 THE EPIDERMIS 



very instructive in showinji' various stages in tlie ontogeny of 

 these hairs. Tlie Jeaf-blade of the sycamore or buttonball tree 

 {Platanus) likewise will provide examples of dendroid multi- 

 cellular hairs. 



2. Scales. Scrape a small (juantity of trichomes from the leaf 

 of Sheplwrdia or Elaeagnus into a drop of water on a slide and 

 examine under low magnification. In Shepherdia two extreme 

 types of peltate trichomes will be seen, viz.: (1) Stellate hairs, 

 consisting of a delicate stalk bearing ten or more distinct radiat- 

 ing unicellular "branches," and (2) Scales which are gray-yel- 

 lowish brown in color, lobate and consist of a plate of many cells. 

 Note under high magnification that not all of the cells composing 

 the disc-like scale radiate from a common center. According to 

 the recent ontogenetic studies of Cooper (1932) this condition 

 results from niiequal arid oblique divisions which may occur near 

 the outei" end of certain of the cells early in the development of 

 the scale. 



3. Colleters. The bud scales of the horse-chestnut (Aesculus 

 Hippocastaniim) ])rovide excellent material for a study of typical 

 colleters. Cut thin trans-sections of the inner scales of a winter 

 bud and mount them in water. Under low magnification note 

 that the abaxial surface of the scale in particular is densely 

 covered with colleters. Additional or alternative material for 

 a study of colleters is furnished by the bud scales of various 

 species of Bhododendron. 



4. Water vesicles. Obtain several thin trans-sections of the 

 petiole of the "Ice-Plant" {Mesemhryanthemuin crystallinum) 

 and examine them in water under low magnification. The adult 

 water vesicle appears as a large clear hemisjiherical cell which 

 projects outwardly from the genei-al eindermal surface. Under 

 high nuignification, a nucleus, scanty cytoplasm and small plas- 

 tids may be detectable in the bladders. If the concave adaxial 

 surface of the petiole of immature leaves is examined, various 

 stages in the origin and expansion of the water vesicle may be 

 seen. 



VII. Suggested Drawings and Notes, — 



]. The nniseriate cpid(riiiis. Prepai-e carefully labeled draw- 

 injis to show the sti'ucturc. in both surface and trans-sectional 



