IX] ANATOMY OF EQUISETUM. 249 



eventually escape by the longitudinal dehiscence of the spor- 

 angial wall. The opening of the sporangia is probably assisted 

 by the movements of the characteristic elaters formed from the 

 outer wall of each spore. 



The spores, which are capable of living only a short time, 

 grow into aerial green prothallia, 1 — 2 cm. in length; these have 

 the form of irregularly and more or less deeply lobed structures. 

 On the larger and more deeply lobed prothallia the archegonia 

 or female reproductive organs are borne, and the smaller or 

 male prothallia bear the antheridia. On the fertilisation of an 

 egg-cell, the Equisetum plant is gradually developed. For a 

 short time parasitic on the female prothallium or gametophyte, 

 the young plant soon takes root in the ground and becomes 

 completely independent. 



As seen in transverse section through a young stem near the 

 apex, the axis consists of a mass of parenchyma, in which may 

 be distinguished a central larger-celled tissue, surrounded by a 

 ring of smaller-celled groups marking the position of a circle of 

 embryonic vascular strands. In each young vascular strand, a 

 few of the cells next the pith may be seen to have thicker walls 

 and to be provided with a ring- like internal thickening ; these 

 have passed over into the condition of annular tracheids and 

 represent the protoxylein elements. At a later stage, a 

 transverse section through the stem shows a central hollow 

 pith, formed by the tearing apart and subsequent disappearance 

 of the medullary parenchymatous cells, which were unable to 

 keep pace with the growth in thickness of the stem. The pith 

 cavity is bridged across at each node by a multi-layered plate of 

 parenchyma, which forms the so-called nodal diaphragm. The 

 inner edge of each vascular strand is now found to be occupied 

 by a small irregularly circular canal (fig. 52, C, c, and D, c) in 

 which may be seen some of the rings of protoxylem tracheids 

 (D, a) which have been torn apart and almost completely 

 destroyed. These canals, known as carinal canals, have arisen 

 by the tearing and disruption of the thin-walled cells in the 

 immediate neighbourhood of the protoxylem. Each carinal 

 canal is bounded by a layer of elongated parenchymatous cells 

 which form part of the xylem of the vascular bundle, and is 



