The first division under the subkingdom Embryobionta, in the classificatory system of A. Cronquist, A. Takhtajan, and W. Zimmermann. The bryophytes and vascular plants are included in this subkingdom. The category Rhyniophyta was devised for the relatively simple Silurian-Devonian vascular plants long held to be ancestral to other groups of vascular plants and usually referred to as Psilophytales. These plants have leafless stems and lack roots; their general morphological structure is not complex. Nevertheless, studies have shown that they fall into distinct groups on the basis of such characteristics as maturation of primary xylem, complexity of primary xylem in the stem, position of sporangia (terminal versus lateral), shape of sporangia (fusiform versus reniform), dehiscence of sporangia (longitudinal versus distal), and presence or absence of extensive lateral branch systems bearing large masses of terminal sporangia. The three classes of Rhyniophyta currently recognized are Rhyniopsida, Zosterophyllopsida, and Trimerophytopsida.