of Vascular Strands in Angiopteris evecta . 395 
strand becomes interrupted as the result of th& exit from it 
of the leaf-traces. Taking a ‘medullated monostele 3 as the 
simplest case, the xyiem at this period forms a cylinder of 
wood enclosing a pith or axile core of parenchyma, and is 
itself ensheathed in a continuous ring of phloem outside 
of which the endodermis is situated. For a time the gaps 
due to the exit of the vascular elements of the leaf-trace 
continue to be repaired, but sooner or later the reparation 
becomes insufficiently rapid, and a differentiation of phloem 
may proceed to extend round the edges of the gap and coat 
the inner face of the xyiem. This may be, and usually is, 
accompanied by a similar differentiation of endodermis. 
Henceforth, the central parenchyma is no longer regarded 
as belonging to the stele, although it may continue to occupy 
the same position in the vascular cylinder taken as a whole, 
but is considered as now belonging to the cortex of the stem. 
Essentially it is the ‘ extension 5 on the part of the phloem 
and endodermis, but especially the latter, which has made 
the difference. The various forms of polystely, dialystely, 
schizostely, with their correlatives gamostely and gamodesmy, 
merely form variants of the process depending respectively 
on the degree of union or separation of the vascular con- 
stituents. 
It may happen, however, that the development proceeds on 
somewhat different lines, and that within the vascular cylinder 
phloem or endodermis, or both, may be differentiated within 
the xyiem from the whole or the peripheral portion of the 
axile parenchyma. In this way are produced the various 
phases of siphonostely 1 and solenostely 2 , which* however, 
become indistinguishable from the other above-mentioned 
types of stelar arrangement, as soon as the continuity of the 
tissues has been broken by the foliar gaps. 
Jeffreys conception of the siphonostele 8 was designed to 
1 In the sense of the term as employed by Jeffrey. 
3 Cf. G wynne- Vaughan, Observations on the Anatomy of Solenostelie Ferns. 
Annals of Botany, xv, p. 73; 
3 E. C. Jeffrey, The development, structure, and affinities of the genus Equisetum. 
Mem. of the Boston Soc. of Nat. Hist., v, p. 160. 
D d % 
