GROUPS OF TISSUES, OB TISSUE-SYSTEMS. 55 



ondary fibro-vascular bundles. When a living portion of 

 a plant is injured, as by cutting, the uninjured cells beneath 

 the wound often change into a layer of cork-cambium, from 

 which a protecting mass of cork is then developed. 



105. A little cork-cambium sometimes forms immediately 

 beneath a breathing-pore, and produces a little mass of cork 

 which pushes out and finally ruptures the epidermis, form- 

 ing what are called Lenticels (Fig. 34). Lenticels are of 

 frequent occurrence on the young branches of birch, beech, 

 cherry, elder, lilac, etc., and may be distinguished by the 

 naked eye as slightly elevated roughish spots, usually of a 

 different color from the epidermis. 



Practical Studies. — (a) Make cross-sections of the stem of the 

 pumpkin. Note that the fundamental portion contains only soft and 

 thick-angled tissues, 



(i) Make a similar section of milkweed (Asclepias) stem. Note 

 that the fundamental portion contains soft, thick-angled, and milk 

 tissue. 



(c) Make cross and longitudinal sections of the leaf of the Scotch 

 or Austrian Pine. Note the fibrous tissue in the hypodermal portion. 



(d) The stone-cells in the pith of the apple-twig are good examples 

 of this tissue in the fundamental system. 



(«) Examine the cells which make up the medullary rays of the old 

 wood of the oak or beech. They will be found to be stony tissue. 

 In young wood they are thin-walled and thus constitute soft tissue 

 (parenchyma). 



(/) Make very thin sections (in different planes) of commercial cork 

 (the product of the Cork-oak of Southern Europe) and mount in alco- 

 hol to expel the air-bubbles. Note the thin walls and the approxi- 

 mately cubical shape of the cells. 



(g) Make very thin cross-sections of a young twig of the apple, 

 snowball, or birch, so as to cut through a young lenticel. Mount in 

 alcohol as before. 



106. Intercellular Spaces. — In addition to the cavities 

 and passages which are formed in the plant from cells and 

 their modifications, there are many important ones which 



