PTEEIDOPHYTA— EQUISETIN^ 



153 



Fig. 913. 



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chymatous tissue, forming a kind of pith. In some it is a large 

 air- cavity. 



There is considerable development of sclerenchymajust under 

 the epidermis, forming a very strong hypoderma. 



Each stele consists of a single closed collateral bundle, the 

 wood of which is much reduced. Generally it consists of the 

 protoxylem and two groups of 

 tracheids {fig. 912, x). In the 

 region of the wood it contains 

 always a conspicuous air space, 

 the carinal cavity, into which 

 the elements of the wood often 

 protrude (fig. 912, I). 



The sub -aerial shoots differ 

 from the rhizome by always 

 having a very large central 

 cavity, extending as before 

 through the several internodes 

 and being interrupted at the 

 nodes. The arrangement of the 

 steles is the same as in the 

 underground stem. The cortex 

 shows similar lacunae, but it 

 contains a number of bands of 

 tissue with chloroplastids, which 

 are situated opposite to the fur- 

 rows. The sclerenchjTua is 

 developed in strands opposite to 

 the ridges. The epidermis con- 

 tains in its cell-walls copious 

 deposits of silica. 



The vascular bundles pursue 

 a straight course down the inter- 

 nodes. At every node, each 

 bundle bifurcates and the two 

 halves diverge ; each half unites 



with half of the next laterally placed bundle, the conjoint bundle 

 passing then down the next internode in the same manner. At 

 each point of junction of two half -bundles a strand from a leaf 

 joins the united portions {fig. 913), so that the bundles are 

 common. 



The leaf is very rudimentary, having only a single small 



Fig. 913. Diagram of the course of 

 the fibrovascular bundles of Equi- 

 setum through two nodes and one 

 internode. 



