Sentence:  And in the <WSD>pleroma</WSD> of the primary meristem of roots there is not only cambium (persistent parenchyma) and procambium (forerunner of fibres and vessels), but pericambium -- i.e., a special outer layer of the <WSD>pleroma</WSD> that remains for a long time as meristem. 
Meaning of the word marked with <WSD> tags:  The central portion of the primary meristem.