OUTGROWTHS AND EXCRETIONS OF THE EPIDERMIS 



67 



Outgrowths and Excretions of the Epidermis. — Excretions 

 of wax in the form of rods, scaleis, and grains often occur over 

 the epidermis of fruits, leaves, and tender stems (Fig. ;^2i)- These, 

 as experiments show, materially assist the water-proofed outer 

 wall in preventing loss of water. 



Outgrowths of the epidermal cells in the form of hairs, both 

 slender and spinous, and scales, are of frequent occurence. In 

 forming these the epidermal cells may- 

 grow outward without undergoing cell 

 division, or they may undergo cell 

 division and give rise to multicellular 

 protuberances. These are apt to Vary 

 in different species in size, form, com- 

 plexity, and other characteristics (Fig. 

 22i), and are very useful to the micro- 

 scopist in defecting the source and 

 purity of powdered foods and drugs. 

 Although in many instances these 

 structures appear to be mere caprices 

 of growth without any function 

 assigned to them, yet, in other cases, 

 their use is very apparent. They are, 

 indeed, employed in a wide range of 

 service, being used as a protection 

 against injury, to assist climbing 

 plants in holding on to their support, 

 in the scattering of fruits, in the re- 

 duction of transpiration and the 



intensity of illumination, in the secretion of special substances; 

 and in the absorption of water and other materials. 



All of those hairs and scales that are more or less rigid and 

 rough, sharp-pointed or barbed, offer difficulties to browsing 

 animals that would tend to lessen their onslaughts. In the 

 stinging hairs of the Urticas the device for protection has reached 

 a high degree of efficiency (Fig. 33). Here the outer wall is 

 silicified about the apex, and is so thin and brittle that it breaks 



FiG- 33- — Different forms of 

 epidermal outgrowths, i, hooked 

 hair from Phaseolus multifiorus ; 

 2, chmbing hair from stem of 

 Humulus Lupulus; 3, rod-like wax 

 coating from the stem of Saccha- 

 rum officinarum; 4, climbing hair 

 of Loasa hispida; 5. stinging hair of 

 Urtica urens. (Fig. 3 after de Bary ; 

 the remainder from Haberlandt.) 



