The structure & development of the mosses and ferns (Archegoniatae) . ylem andphloem is a well-defined layer of cambium by whose growththe thickness of the vascular cylinder is slowly but constantlyadded to, and as a result there is a secondary growth of thestem strictly comparable to that of the Dicotyledons. The outer layer of the cortex (the epidermis is quiteabsent) develops cork, but not from a definite cork cambium.^These cork cells arise by repeated tangential divisions in cells Holle (i), p. 249. 246 MOSSES AND FERNS CHAP. near the periphery, and have in consequence the same regulararr
The structure & development of the mosses and ferns (Archegoniatae) . ylem andphloem is a well-defined layer of cambium by whose growththe thickness of the vascular cylinder is slowly but constantlyadded to, and as a result there is a secondary growth of thestem strictly comparable to that of the Dicotyledons. The outer layer of the cortex (the epidermis is quiteabsent) develops cork, but not from a definite cork cambium.^These cork cells arise by repeated tangential divisions in cells Holle (i), p. 249. 246 MOSSES AND FERNS CHAP. near the periphery, and have in consequence the same regulararrangement seen in similar cells of the higher plants. A cross-section of the petiole of the earliest leaves of theyoung plant show but a single nearly central vascular bundle,but as the plant grows older the number becomes much larger,and may reach ten.^ In leaves of moderate size there areusually about four, and these are arranged ground tissue is composed mainly of large thin-walledparenchyma and a well-marked epidermis. The fibrovascular n • n.
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