GROUPS OF TISSUES, OR 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, ete., 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. 
(6) 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. 
(@) 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 beech. They will be found to be stony tissue. 
In young wood they are thin-walled and thus constitute soft tissue 
(parenchyma). 
(f) 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 
