THE CUTICLE. 85 



connected with the cells containing the acrid secretion ; and 

 by the force required to pierce the skin it presses upon the 

 cavities which propels the lluid up the tuhe, and injects it into 

 the wound made by the point. It is this poison whicli causes 

 the severe pain occasioned by the sting of the nettle. 



4. Prickles are hard, sharp pointed, stiif productions of 

 the cuticle, often hooked at the extremities. When the 

 prickles have acquired their full growth, they arc quite firmly 

 attached to the stem ; but as the stem advances in size, the 

 prickles, remaining of the same dimensions, become loosened 

 at their base and fall off. Hence, old stems are seldom cov- 

 vered with prickles, while the younger ones are prickly. 



5. Scurf, or Lcpides, appearing to the naked eye like a 

 mealy substance on some leaves, are scales attached to the 

 stem by their center, and seem to be formed by the cohesion 

 of many hairs having the same point in the cuticle for their 

 origin. 



6. Lenticcls are brown spots appearing on the stems of 

 many trees and shrubs, at tirst nearly round, but as the stem 

 increases in size, they assume a linear form, and produce 

 transverse spots on the surface of the stem, as may be seen 

 in the cherry, willow, birch, and other trees and shrubs. By 

 a closer examination, we find the Lenticels to consist of a 

 corky substance apparently projecting through apertures in 

 the cuticle and being divided into two lips by a medial slit. 

 By cutting through one of these lenticels transversely, and 

 examining it by a microscope, the student will find that they 

 are placed on the external layer of the bark, between it and 

 the cuticle, and that it has no connection with the bark, much 

 less with the wood. Hugo Muhl has demonstrated that these 

 productions, generally at least, have their origin in cells lying 

 between the cuticle and bark, and arranged in regular series 

 perpendicular to the axis of the stem ; and that they are 

 smaller than the cells of the subjacent parenchymous laver 

 of the bark, and that by an opening in the cuticle they be- 

 come exposed to the air, and develop themselves in the dry, 

 brown, corky mass, denominated lenticels. 



40. From the above description, one would not expect that 

 they could form a subject of much interest, but it would re- 

 quire volumes to contain the discussions of the learned on 

 the functions of lenticels. Do Candolle has asserted that 

 they are the origin of adventitious roots. This opinion was 

 founded on the fact that when a piece of willow was placed 

 in water, the root protruded, through the lenticels, but it has 



