THE OBOUPS OF TISSUES, OB TISSUE SYSTEMS. 61 



a plant is injured, as by cutting, the uninjured cells be- 

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

 from which a protecting mass of cork is then developed. 



107. A little cork-cambium sometimes forms immedi- 

 ately beneath a breathing-pore, and produces a minute 

 mass of cork which pushes out and finally ruptures the 

 epidermis, forming Lenticels (Fig. 39). 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 tlie stem of tlie 

 pumpkin. Note that the fundamental portion contains soft, fibrous 

 and thiclt-angled tissues. 



(b) 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. 



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

 wood of the oak or beecTi. 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. 



108. 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 

 are intercellular and which at no time were composed of 

 cells. In some cases they so closely resemble the cavities 

 derived from cells that it is with the greatest difficulty that 



