308 ME. W. C. WOESDELL ON " TEANSFUSION-TISSUE." 



transfusion-tissue at the end of the phloem. In this group of tracheides could be 

 observed, both in transverse and longitudinal sections, a most interesting transition in 

 the character of the elements. Starting from the large tracheides of the main part of 

 the centripetal xylem, we first come to the small tracheides of the connecting group, 

 which approximate in size to the tracheides of the centrifugal wood, and pass from these 

 to the elements of the transfusion-tissue, in which, at first, a gradual thickening of the 

 walls can be noticed, then a shortening in length and an increase in diameter of the 

 elements, until they merge into the ordinary parenchyma-shaped tracheides which form 

 the bulk of the transfusion-tissue (PI. XXIII. fig. 2). These tracheides sometimes 

 occur among the stone-cells of the endodermal ring ; in other cases it appears 

 that communication takes place between the tracheides in the pericycle and those 

 in the mesophyll through a stone-cell of the endodermis, which (and this is invariably 

 the case) at this point has no contents, while the stone-cells on either side contain 

 numerous crystals. 



But I now have to deal with a tissue in the leaf of this plant which is unique among 

 Cycads. Stretching amidst similarly elongated cells in the central region of the 

 mesophyll of the leaf, from the midrib to the margin, is a band of conspicuous tracheides, 

 narrow and elongated in shape and with bordered pits on their walls. These bordered 

 pits are mostly of a rather rudimentary type, with wide slit-like pores, but in many of 

 the tracheides nearest the vascular bundle they are more typical in appearance. These 

 tracheides are in intimate connection, by means of their wide, expanded ends, with the 

 small tracheides of the transfusion-tissue. They form a loose complex of tissue extending 

 along the whole length of the leaf in a plane parallel to its two surfaces (PI. XXIV. fig. 11). 

 Lignier * has attempted to account for this tissue by regarding it as the remnant of a 

 lateral system of dichotomizing bundles such as that found in the leaf of Stangeria. He 

 appears to offer no special reason for this assumption, but founds it merely on the general 

 structure and appearance of these strands of tracheides. Now, to my mind, the general 

 arrangement and structure of this tissue are precisely the characters which prevent any 

 such origin as that proposed by Lignier being attributed to it. It is almost impossible 

 to suppose that the regular venation of a leaf like that of Stangeria, with distinct and 

 conspicuous intervals between the veins, could have been transformed into the irregular 

 strands of anomalous tracheides which occupy the entire middle portion of the leaf, with 

 the exception of the mesophyll-cells, narrow and elongated like themselves, which lie 

 scattered among them. Ear more probable does the explanation seem that these 

 tracheides are a later modification of the mesophyll-cells of the leaf, which has been 

 adopted since the narrowing of the leaf and the loss of the lateral veins. Lignier also 

 considers the normal transfusion-tissue, as found in Gymnosperms generally, to be a 

 remnant of such a lateral system of venation. That this idea is equally false I hope to 

 show clearly before the close of this paper. If it can be proved that the normal 

 transfusion-tissue is in no wise such a remnant as he supposes, but is an independent 

 formation, then this would be an additional proof that the anomalous trans fusion -tissue 



* Bulletin de la Soc. Linn, de Normandie, ser. IV. tome vi. fasc. 1. 



