Spruce needle. Coloured scanning electron micrograph (SEM) of the surface of a needle of a sitka spruce (Picea sitchensis). The sunken pits contain closed pores called stomata. Each stoma can be opened and closed by the two guard cells (light green) on either side of it. This allows the exchange of gases between the plant and the air to be controlled. The sunken stomata and the layer of wax (pale green) covering the surface of the needle minimise water loss from the tree. This conifer is native to North America, especially the north-west. Magnification: x380 at 6x7cm size.