=The Color of the Gills.=--The color of the gills of the common mushroom varies in different stages 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 _sterígmata_ (sing. _sterígma_), 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.)]