Monostele

Again, in ectophloic medullated monosteles the presence of an internal endodermis between xylem and central parenchyma is rare ; on the other hand when an amphiphloic condition obtains, an endodermis often cuts off the internal phloem from the central parenchyma.

Source: wiktionary

This is most readily done by reference to the sporeling: for there the axis is found to be traversed by a coherent monostele: and it is to the disintegration of this, together with the formation of commissural connections, that the complex structure of the adult is due.

Source: wiktionary

This condition be conceived to be derived by repeated division of the monostele.

Source: wiktionary

'Monosteles' comprising a ring of alternating xylem and phloem elements surrounding parenchymatous pith; 'monosteles' occurring in most genera...

Source: wiktionary