THE TISSUES OF PLANTS. o7 
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. 
(0) 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, aud basswood, and note the differences 
in the amount and compactness of their fibrous tissue. 
(f) 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 of the milk (latex) under the microscope by trans- 
mitted light. When 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 thin longitudinal sections of the stem of a Milkweed 
(Asclepias). By careful searching, tubes containing latex (appearing 
light granular brown) may be seen. 
(0) 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, 
(c) The more complex or reticulated forms of milk-tissue may be 
obtained from the stems of wild lettuce, garden-lettuce, poppy, and 
blood-root. 
(@) 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-Tissue.—As sieve-tissue is always found in the woody 
bundles which run lengthwise through the higher plants, it is neces- 
