59 8 BOTANY OF THE LIVING PLANT 



first small but often attaining considerable size in the adult (Figs. 140, 374). In 

 all advanced Ferns the stelar tissue is shut off from the pith by an inner 

 endodermis, giving the cylindrical structure known as solenostelic (Fig. 452). 

 Further the tube is apt to be interrupted by foliar and other gaps, so that 

 it appears as a network of meristeles surrounding the column of pith. This 

 allows of ventilation inwards, a matter of some importance where the column 

 is large, as it is in the Shield Fern (Fig. 374). In many Tree Ferns, such as 

 Dicksonia, the pith may measure several inches in diameter. At first it consists 

 of relatively inert storage parenchyma, without any conducting tissue to aid 

 transit within its bulk. But this physiological difficulty has been met in many 

 large Ferns by the formation of accessory conducting tracts that are present 

 in the pith. An example of this has already been seen in the inner meristeles 

 of the Bracken (Fig. 376). These medullary strands often take the form 

 of concentric rings, as in Pteris podophylla (Fig. 453) • Their number increases 

 with the size of the stem. In the largest (6) three complete rings are seen 

 and the inception of a fourth : and in extreme cases as many as a dozen 

 may be found, all fitting concentrically within one another. This has been 

 seen in the fossil genus Psaronius. In others, however, as in Platycerium, 

 isolated strands appear dotted over the transverse section, as in the stems of 

 Monocotyledons (Figs. 29, 30). 



In the very diverse vascular types of Ferns and Monocotyledons the con- 

 ducting system of each vegetative shoot forms a connected whole, and each is 

 of primary origin only. The stelar expansion of each is correlative with the 

 obconical enlargement towards the apical bud. The stolons of Nephrolepis 

 give a clear demonstration of the relation of stelar complexity to size. Since 

 both stolons and tubers are leafless, the modifications of form of the vascular 

 strands are not affected by leaf-insertions, but are determined in the growing 



bud itself. 



Where the size of the stolons in Nephrolepis is small the form is cylindrical, 

 and it is traversed by a solid stelar core. But where the stolon itself swells 

 distally into a pear-shaped tuber the stele may become variously fluted, or even 

 disintegrated, as it is in Nephrolepis cordifolia (Fig. 454, A and B). At its base 

 the solid stelar core expands first into a solenostele, and later disintegrates 

 into a ring of meristeles (C). But as it passes upwards into the conical tip it 

 gradually re-constitutes itself ; and the protostelic state is resumed (Fig. 

 454, A). The conclusion from such facts is that these converse changes, 

 basal and apical, are related locally to the dimensions of the tuber. Such 

 facts accord with what is seen in the leafy shoots of Ferns at large (Figs. 374, 

 375, 376, 376.^, 455). Both of these phenomena are presented by primary 

 tissues defined at the growing point itself. 



In a vertical section of the sporeling of Polypodium vulgare (p. 208, Fig. 140) 

 the apical region appears broad and flattened, and the leaves arising from it 

 are solitary. But it is a relatively small plant, its apical region being in dia- 

 meter about ^„ th that of the adult Tree Fern shown in surface view in Fig. 450. 

 Here the young leaves are crowded round the growing point, each requiring its 

 own quota of nourishment. The problem of supplying all the leaf-primordia 

 may be held as particularly acute, at the time when a new succession of them 

 is developing. In various degrees they will call for supplies from the expand- 

 ing shoot through each leaf-base. Actually very little is known of the mode of 



