1 68 



THE VASCULAR SYSTEM OF THE LEAF 



[CH. 



down the petiole of Plagiogyria semicordata, though in P. pycnophylla there 

 are none. The meristele is thus divided into three as seen in transverse 

 section. The sHts close downwards before the enlarged leaf-base is reached. 

 Such slits may be considered to be of the nature of " perforations," allow- 

 ing interchange directly through the vascular tract {Studies, I, p. 431). 



These illustrations are taken from Ferns of moderate size. But where 

 the leaf is large the vascular supply must also be large. A simple expansion 

 of the trace into a widely arched 

 curve is found in many such 

 cases (Fig. 161, i, 3). The num- 

 ber of the protoxj'lems increases 

 and the whole horse-shoe ap- 

 pears thus to be made up of a 

 number of units linked into a 

 continuous chain. Each unit 

 has a protoxylem-group as its 

 centre, from which metaxylem 

 extends right and left until the 

 units are linked together to 

 form a continuous tract, sur- 

 rounded by phloem, pericycle, 

 and endodermis. Bertrand and 

 Cornaille gave the name of 

 "divergent" to these units, and 

 they have applied an elaborate analysis of the leaf-trace from this point 

 of view to Fern-leaves generally. Such expanded curves are seen in the 

 petioles of Metaxya among the Superficiales, and in Thyrsopteris among 

 the Marginales (Fig. 161, 3, 5). 



The expansion of a continuous vascular sheet with increasing size raises 

 difficulties of interchange between cortex and central parenchyma. They 

 are met sometimes in large leaves by corrugation of the vascular tracts, the 

 protoxylems lying at the crests of the folds while the arches of metaxylem 

 curve convexly inwards (Fig. 161, 3). The effect of this is to enlarge the 

 surfaces of the vascular barrier, and so to facilitate interchange. But 

 commonly the curve itself is also thrown into deep folds. As the arched 

 leaf-trace of a large leaf passes into the petiole it expands, and two deep 

 lateral involutions appear, which have obvious relation to the two ventilating 

 tracts at the outer surface (Fig. 161, 5, 6//). In most Ferns of large size 

 the petiole is strengthened by peripheral sclerenchyma impervious to gas- 

 interchange. But laterally two lines, or in some cases interrupted areas 

 roughly in linear succession, run up the leaf-stalk, and onwards to the 

 lamina, where they are superseded by the flange-like wings of the flattened 



Fig. 161. Transverse sections of petioles, all drawn to the 

 same scale. (X2.) i, Dipteris conjugata; 2, Dipteris 

 Lobbiana; 3, Metaxya; 4, Pklebodmm aureiim; 5, 

 Thyrsopteris; 6, Alsopkila australis. These show that 

 while greater size leads to vascular disintegration, there 

 is no definite proportion. 



