THE TISSUES OF PLANTS. 27 



that some of the cells have thickened walls. These are very hard, 

 and are to be regarded as a form of stony tissue. They contain starch. 



IV. Fibrous Tissue. — (a) Split a young maple-twig, then with the 

 scalpel start a thin longitudinal radial section, completing it by tear- 

 ing it off. Mount in water. The torn end will show good wood- 

 fibres. 



(J) Make a very thin cross-section of the wood of the same twig, 

 using a very sharp scalpel. Note the angular shape of the wood- 

 fibres in this section. 



(c) Make a cross-section of the bark of the same twig and note the 

 white bundles of bast-fibres, each fibre having greatly thickened 

 walls and a very narrow cell-cavity. 



(d) Now make several longitudinal sections of the same twig so as 

 to cut through one of the bundles of bast-fibres. Note the great 

 length of the bast-fibres. 



(e) Make cross-sections of the wood of various trees, as oak, hick- 

 ory, elm, ash, poplar, willow, and basswood, and note the differences 

 in the amount and compactness of their fibrous tissue. 



(/) To isolate the wood-fibres, make a number of sections as in (a) 

 above, then heat for a minute or less in nitric acid and potassium 

 chlorate. The fibres may now be separated under a dissecting micro- 

 scope, or the specimens may be transferred to a glass slide and dis- 

 sected by tapping gently upon the centre of the cover-glass. This is 

 known as Schulze's maceration. 



V. Milk-Tissue. — In studying milk-tissue it is necessary first to 

 examine a drop ot the milk (latex) under the microscope by trans- 

 mitted light. Wlien so examined it presents quite a different appear- 

 ance from that by ordinary reflected light: thus white latex appears 

 to be light granular brown. 



(a) Make tliin longitudinal sections of the stem of a Milkweed 

 (Asclepias). By careful searching, tubes containing latex (appearing 

 light granular brown) may be seen. 



(jb) Make a similar study of the stem of the large Spurge (Euphor- 

 bia) of the greenhouses. Its milk-tissue is thick-walled and easily 

 made out. 



(e) The more complex or reticulated forms of milk-tissue may be 

 obtained from the stems of wild lettuce, garden-lettuce, poppy, and 

 blood-root. 



(d) Collect a quantity of latex of a Spurge or Milkweed in a watch- 

 glass and slowly evaporate it: the residue will be found to consist of 

 a sticky, elastic material resembling india-rubber. 



VI. Sieve-Tisstie.— As sieve-tissue is always found in the woody 

 bundles which run lengthwise through the higher plants, it is neces- 



