tages of development. When very young the gills are
white. But very soon the gills become pink in color, and during the
button stage if the veil is broken this pink color is usually present
unless the button is very small. The pink color soon changes to dark
brown after the veil becomes ruptured, and when the plants are quite old
they are nearly black. This dark color of the gills is due to the dark
color of the spores, which are formed in such great numbers on the
surface of the gills.

[Illustration: FIGURE 8.--Agaricus campestris. Section of gill showing
_tr_==trama; _sh_==sub-hymenium; _b_==basidium, the basidia make up the
hymenium; _st_==sterigma; _g_==spore. (Magnified.)]

=Structure of a Gill.=--In Fig. 8 is shown a portion of a section across
one of the gills, and it is easy to see in what manner the spores are
borne. The gill is made up, as the illustration shows, of mycelium
threads. The center of the gill is called the _trama_. The trama in the
case of this plant is made up of threads with rather long cells. Toward
the outside of the trama the cells branch into short cells, which make a
thin layer. This forms the _sub-hymenium_. The sub-hymenium in turn
gives rise to long club-shaped cells which stand parallel to each other
at right angles to the surface of the gill. The entire surface of the
gill is covered with these club-shaped cells called _basidia_ (sing.
_basidium_). Each of these club-shaped cells bears either two or four
spinous processes called _sterigmata_ (sing. _sterigma_), and these in
turn each bear a spore. All these points are well shown in Fig. 8. The
basidia together make up the _hymenium_.

[Illustration: FIGURE 9.--Polyporus borealis, showing wound at base of
hemlock spruce caused by falling tree. Bracket fruit form of Polyporus
borealis growing from wound. (1/15 natural size.)]

=Wood Destroying Fungi.=--Many of the mushrooms, and their kind, grow on
wood. A visit to the damp forest during the summer months, or during the
autumn, will reveal large numbers of these plants growing on logs,
stumps, from buried roots or rotten wood, on standing dead trunks, or
even on living trees. In the latter case the mushroom usually grows from
some knothole or wound in the tree (Fig. 9). Many of the forms which
appear on the trunks of dead or living trees are plants of tough or
woody consistency. They are known as shelving or bracket fungi, or
popularly as "fungoids" or "fungos." Both these latter wo