332 BOTANY OF THE LIVING PLANT 



single layer. Within this conies tha phloem (pli), with large sieve-tubes 

 as the characteristic elements. They are thin-walled, with watery 

 contents. The lateral walls where two adjoin bear the sieve-plates, 

 and are recognised by glistening globules that adhere to them. They 

 are embedded in parenchyma, which extends inwards into the xylem, 

 and may be called collectively conjunctive parenchyma. The chief 

 features of the xylem [xy) are the tracheids, which are relatively large, 

 with a very characteristic polygonal outline. They have woody 

 walls, and no protoplasmic contents. Where two adjoin the walls are 

 flattened, and of double thickness, showing that each has its own share 

 of the thickening, which overarches the pit-membrane as in the pits of 

 Conifers. The structure is in fact essentially the same, only in Ferns 

 the pits are liable to be extended transversely. But where the 

 tracheid abuts on parenchyma-cells the pits are narrower. Inter- 

 nally, and usually about the foci of the elliptical meristele smaller 

 tracheides are found. These are the first-formed tracheides, or 

 protoxyleni. The meristele of a Fern is thus concentric in construction ; 

 it is strictly delimited, and has no provision for increase in size. 



The tracheide of the Fern resembles that of the Pine in bemg of spindle- 

 form, with its thickened hgnified walls marked by bordered pits. But whereas 

 tire pits in the Pine are circular, those in the Fern are liable to be transversely 

 elongated, as is natural in tracheides so wide as these are. Their features 

 are weU seen in longitudinal sections, but better if they are isolated by macera- 

 tion (Fig. 271, A). The elongated pits lie parallel to one another, and this is 

 specially well seen where two wide tracheides have faced one another. From 

 the ladder-like appearance that resu.ts they have been called scalariform 

 tracheides. Examined under a high power the double outline of the pits is 

 seen, and when the pits are small and circular the similarity to those of the 

 Pine is plain (Fig. 271, B). In most Ferns the pit-membranes persist, but 

 in Pteridium they appear to be liable to be broken down, and the cavities 

 thrown together technically as in vessels. The tracheides of 'he protoxylem 

 are seen in longitudinal section to be spiral or reticu-ate, as in other Vascular 

 Plants. 



The sieve-hibes are also spindle-shaped, and are without companion-cells. 

 Their cellulose walls are swollen. Where two sieve-tubes adjoin, numerous 

 thinner sieve-areas of irregular outline are borne. They are found to be 

 perforated by very fine protoplasmic threads extending between highly 

 refractive globules that adhere to the walls (Fig. 272). Such tracheides and 

 sieve-tubes are characteristic of Ferns, and with differences of detail, of other 

 Pteridophytes as well. 



The anatomy of the leaf in Ferns resembles that of Seed-Plants down even 

 to the collateral structure of the vascular strands. Bemg chiefly shade- 

 loving plants chlorophyll is usually present in the cells of their epidermis, 

 and the differentiation of the mesophyll into palisade and spongv parenchyma 

 is not marked. In these respects they resemble the leaves of Angiosperms of 



