VII] 



INTERNAL ENDODERMIS 



131 



other tissue of like nature. An internal endodermis is even to be found sometimes sur- 

 rounding the pith in the root of Helminthostachys^ a position in which endodermis is nor- 

 mally unknown. Here at least it seems clear that the inner endodermis is a new development, 

 and the same is probably true also in all the stems named (Lang, A7i7t. of Bot. xxix, p. 6, 

 Text-Fig. I B). Since the plants in which sporadic inner endodermis occurs are all 

 relatively primitive, the natural way to view them is as steps in advance rather than as 

 relics of degeneration : and to see whether 

 there may not be some physiological reason 

 for its presence. 



The most instructive examples of the spo- 

 radic internal endodermis are in Botrychium 

 and Helminthostachys, which have been care- 

 fully worked over by Lang {Ann. of Bot. 

 xxvii, p. 203, xxix, p. i). In the young plant 

 oi Botrychiwn Luna7'ia it is in the region in- 

 termediate between the juvenile and the adult 

 parts that the internal endodermis is found ; 

 it is associated with extension of the inter- 

 nodes, and interruptions of the outer endo- 

 dermis at the exit of the leaf-trace. Here, as 

 Lang has shown, the pith is liable to have 

 open connection with the cortex. The internal 

 endodermis would then be a useful newly 

 organised protection for the conducting tract, 

 its function being to place the contents of the 

 stele under protoplasmic control, though still 

 allowing of gaseous interchange through the 

 foliar gap between the ventilated pith and the 

 cortex. In the adult region, with its greater 

 bulk and shorter internodes, this is less im- 

 portant, and there the internal endodermis 

 is absent. Figs. 124 A, B show its distribu- 

 tion in two fully analysed examples, and how 

 in a long internode the inner endodermis may 

 form a deep pocket-like tube. But Lang 

 states that in Botrychiiim such pockets are 

 never closed at the base. In Helmintho- 

 stachys., in which the internodes are short, 

 Lang specially notes that it is only in the 

 adult rhizomes with larger leaf-gaps that 

 continuitybetween the cortical and medullary 

 parenchyma is marked, and it is in these 

 regions that an internal endodermis is often 



Fig. 1-24. Botrychium Lu7taria. ^ = Reconstruc- 

 tion of the stelar structure by Lang of his plant 

 E. 8 = 2. similar reconstruction of his plant F. 

 Only the xylem and the endodermis rire indi- 

 cated, the former black; the latter as a line 

 where seen in section; but dotted where the 

 internal endodermis is seen in surface view, 

 as if the stele were split in half. The leaves 

 are really arranged spirally, but are here re- 

 presented as if they arose alternately. The level 

 of origin of the root-traces is indicated. The 

 proportions have been altered from those in 

 nature so that the stele is represented as broader 

 in proportion to its length than is actually the 

 case, (x) represents axillary buds ; (*) the apex. 



developed. It thus appears that the inner endodermis has a use which would justify its ap- 

 pearance as a new formation. 



The structure oi Platyzoma has been worked out in detail by Thompson {Trans. Roy. 

 Soc. Edin. lii, 1919, p. 571). In fully developed specimens of this xerophytic Fern there is 

 a medullated stele with a continuous ring of xylem, from the outer margin of which leaf- 

 traces arise without any interruption of the ring. The medulla is often heavily sclerosed, and 

 between this and the xylem there is a ring of inner endodermis, which is at no point con- 



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