SHOWS MICROPHYLLY TO BE PRIMITIVE 199 



axis is protostelic at the base, and' the first leaf-trace may depart without in 

 any way altering the structure of this stele, thus showing the cladosiphonic 

 character. It is only subsequently, by the overlapping of those intrusive 

 pockets of phloem and endodermis which accompany the exit of the leaf- 

 trace, that a gradual transition is effected to the 

 phyllosiphonic type. Thus the anatomical evidence 

 indicates a probability that, even in large-leaved 

 Ferns, the cladosiphonic was the primitive type ; 

 but that the phyllosiphonic, once initiated, is as a 

 rule maintained : this is shown by its persistence 

 in the Seed-Plants, even where the leaf has been 

 reduced in size. Accordingly the trend of ana- 

 tomical evidence is towards the recognition of a 

 small-leaved, strobiloid type of construction of the 

 primitive sporophyte. 



It may be objected that in certain Pteridophytes 

 the condition of the embryo is such as to- militate 

 against any strobiloid theory, and that their ana- 

 tomical details offer as great an obstacle as their 

 external form. The cases which will be cited are 

 those of some Lycopods, in which the first leaves 

 are isolated, and show no vascular connection with 

 the later-formed leaves (Fig. 101): or my own 

 observations on Phylloglossum may be held as a 

 valid objection, for there also a single vascular 

 strand has been observed passing down in one case 

 directly from a protophyll into a root, without any 

 relation to the other members. 1 Such cases may 

 be held to establish the individuality of the leaf 

 anatomically before the axis has any existence. In 

 the interpretation of these the discussion on embry- 

 ology in Chapter XIV. should be recalled, and 

 especially the opinion there brought forward that 

 the separation of individual leaves in certain cases 

 from the rest of the shoot is a secondary condition, 

 resulting from what might be called anticipatory 

 development, to meet an early need for an assimi- 

 latory mechanism. The inconstancy of detail which 

 is shown by the embryos of Z. cernuitm, and by the young annual growths 

 of Phylloglossum^ is in itself a support of this opinion. The vascular supply 

 of the leaves thus isolated is separated it is true from that of the main 

 shoot, but its separateness may be held to be secondary, and not a proper 

 basis for conclusions as to the primitive construction of the vascular system 

 of the normal shoot. 



1 Phil. Trans., part ii., 1885, Plate 73, Fig. 42A. 



FIG. too. 



A Isophila excelsa. Diagram 

 of vascular system of a young 

 plant in median longitudinal 

 section. The xylem is black, 

 the phloem lightly shaded, and 

 the endodermis is indicated by 

 a dotted line, the ground-tissue 

 is left white. (After Gwynne- 

 Vaughan.) 



