VIII] 



DICTYOSTELY 



147 



reconstructions such as that made by McLean Thompson {Trans. R. S. 

 Edin. Hi, p. 727, Fig. 5). The Lindsay a-zow^x'dow recurs regularly at an 

 early stage in the ontogeny, and 

 the state seen in Lindsaya may 

 be understood as that where no 

 structural advance has been made 

 to complete solenostely: probably 

 this is owing to the restricted size 

 of the species of this genus, which 

 made distension of the stele 

 unnecessary. The last step to 

 typical solenostely is, however, 

 taken in the larger but allied 

 genus Odontolonia^ which is some- 

 times merged in Lindsaya. 



It has thus been shpwn in the 

 ontogeny of Ferns, both Super- 

 ficiales and Marginales, that a Fig.137. Zm^'^aj/aj-mwaewj. Aseriesofsectionsthrough 

 , • r 1 J 3. node, showing the internal phloem and the mode 



regular succession Ot stages leads of detachment of the leaf-trace. (After Tansley.) 



from the protOStelic to the sole- ^y^*^"^ is hatched, phloem dotted, and endodermis a 



^ simple line. 



nostelic state. They are, the 



protostele, the medullated protostele, the Lindsaya-stdigQ, and finally the 

 solenostele. But the length of each stage may vary, being greatest where 

 the internodes are long and the adult stem is creeping: and shorter where 

 the axis is upright, and the leaf-arrangement closer. 



DiCTYOSTELY 

 A natural consequence of that shortness of the axis which is seen in 

 ascending, or vertical shoots, and of the crowding of the leaves which they 

 bear, is the apparent further complication of the vascular system called Dictyo- 

 stely. It is specially prominent when the stem is seen in transverse section. 

 Instead of the stelar ring being interrupted only at a single point at a node, 

 as in the solenostelic rhizome, several foliar gaps appear in each transverse 

 section (Fig. 138, C, D). This involves no other innovation than the shortening 

 of the internodes, and the consequent overlapping of the leaf-gaps : and it 

 is already suggested in certain creeping rhizomes with their leaves closely 

 set (Fig. 139). But the best indication of it comes in those runners which 

 appear as branches on many Ferns. They have usually long internodes at 

 first, but later the leaves are more crowded on their upturned tips, where 

 the axis is proportionately widened to receive them. This is well shown in 

 Plagiogyria pycnophylla, in which such branches, called stolons, frequently 

 spring near to the leaf-bases (Fig. 138, A — D). At first the axis is simply 



