In the simpler Mosses, the stem consists of a parenchymatous ground-tissue with a few outer layers of thicker-walled and smaller cells. In others there is a distinct central cylinder which occupies the axis of the stem, and consists of long and narrow cells; in the more complex forms the structure of the axial tissues suggests the central cylinder or _stele_ of higher plants. The genus _Polytrichum_, so abundant on English moors, illustrates this higher type of stem differentiation. In a transverse section of the stem the peripheral tissue is seen to be composed of thick-walled cells, passing internally into large parenchymatous tissue. The axial part is occupied by a definite central cylinder consisting in the centre of elongated elements with dark-coloured and thick walls having thin transverse septa; surrounding this central tissue there are thinner walled elements, of which some closely agree in form with the sieve-tubes of the higher plants. The central tissue may be regarded as a rudimentary type of xylem, and the surrounding tissue as a rudimentary phloem. Each leaf is traversed by a median conducting strand which passes into the stem and eventually becomes connected with the axial cylinder. The fertilisation of the egg-cell gives rise to the development of a long slender stalk terminating distally in a large spore-capsule. In section the stalk or seta closely resembles the leafy axis of the moss plant. Considering the fairly close approach of some of the mosses to the higher plants as regards histological characters, it is conceivable that imperfectly petrified stems of fossil mosses might be mistaken for twigs of Vascular Cryptogams.