The part in contact with the endosperm is plate-like, and is known as the scutellum; the surface in contact with the endosperm forms an absorptive epithelium.
When none of the radiations which fall on a body penetrates through its substance, then the ratio of the amount of radiation of a given wave-length which is absorbed to the total amount received is called the "absorptive power" of the body for that wave-length.
Thus if the body absorbed half the incident radiation its absorptive power would be 2, and if it absorbed all the incident radiation its absorptive power would be r.
If the axis of such a sporogonium were prolonged downwards into the soil to form a fixing and absorptive root, the whole structure would become a physiologically independent plant, exhibiting in many though by no means all respects the leading features of the sporophyte or ordinary vegetative and spore-bearing individual in Ptericlophytes and Phanerogams. These facts, among others, have led to the theory, plausible in some respects, of the origin of this sporophyte by descent from an Anthoceros-like sporogonium (see PTERIDOPHYTA).
In the terrestrial plants it differs in the subterranean and subaerial parts, being in the former preeminently absorptive, and in the latter protectiveprovision at the same time being made for the gaseous interchange of oxygen and carbon dioxide necessary for respiration and feeding.