 The tubes which comprise the pore layer on the underside of the cap are 0 @.@ 4 to 0 @.@ 6 cm ( 0 @.@ 16 to 0 @.@ 24 in ) deep , and have an adnate ( attached broadly to the stem ) to decurrent ( running down the length of the stem ) attachment to the stem . They are yellow , and stain reddish @-@ brown when bruised . The yellow pores are large ( 1 – 2 mm diameter ) and angular , and tend to become darker as they age . The pores are slightly wider than long , so that there are about 9 – 10 pores per centimeter measured radially , but 12 to 13 per centimeter when measured tangentially , about halfway to the edge . As is the case with all boletes , spores form on the inner surfaces of the tubes and sift through their openings to be borne away on the air currents outside . 
