VI 1 1] 



SOLENOSTELY 



143 



the base of the young plant there is first a long protostelic tract, which bears 

 a number of leaves. The structure is in fact that which is seen in permanently 

 protostelic Gleichenias. This first stage is followed by a long medullated 

 stage, the central region of the xylem being replaced by parenchyma. At 

 first there are no xylic gaps at the departure of the leaf-traces, but later the 

 xylem-ring is interrupted, and opposite the xylic gaps thus formed there 

 may be a slight involution of the endodermis. At first no internal phloem 

 is seen, but parenchyma only. Just before the solenostelic stage is reached 

 inner phloem appears as an indefinite and incomplete ring of sieve-tubes 

 lining the xylem internally. The solenostelic condition finally results from 



Fig. 133. Diagram illustrating the solenostelic structure, and attachment of the leaf-trace in 

 Gleickeftia pedinata. The transverse sections show the structure corresponding to the several 

 points indicated. After Boodle. 



the following further step, which is described as from below upwards. First 

 an inner endodermis appears, as a tube closed below and widening upwards. 

 It is continuous on the one hand with the endodermis of the next departing 

 leaf-trace : and on the other it is continued as the inner endodermis lining 

 the next higher internode : it is continuous also with the endodermis sur- 

 rounding the next succeeding leaf-trace. These steps are all illustrated in 

 the reconstruction by Dr Thompson (Fig. 134). The contimcity of the endo- 

 dermis right across the pith-column follows on the interruption of the stelar 

 xylem and phloem immediately above the departure of a certain leaf-trace, 

 which is the first showing a true foliar gap. Its effect will be to stop any 

 possible leakage outwards from the otherwise completely delimited vascular 



