SECONDARY XYLEM 665 



not so complete as the first, but is nevertheless quite clearly recognis- 

 able. The smallest amount of deviation from the normal type of 

 wood-fibre, which is a dead structure, is illustrated by fibres that 

 retain their living contents ; storage or translocation of plastic sub- 

 stances can, of course, only take place in cells that are provided with 

 living protoplasts. Such living wood-fibres are perfectly typical as 

 regards their shape and the character of their walls ; but the narrow, 

 oblique (sinistrorse) pits show a tendency to congregate upon the radial 

 walls, in many cases {e.g. in the genus Acer, according to Krah). The 

 next step is exemplified by septate wood-fibres, such as occur in the 

 Vine. Cells of a remarkable type, connecting mechanical elements with 

 " intermediate cells," have been observed by the author in the wood of 

 Cytisus Laburnum, where they occur in considerable numbers at the 

 junctions of the broad plates of wood-fibres with tracts of xylem 

 parenchyma; each of these special cells (Fig. 275 c) exactly resembles a 

 typical thin-walled intermediate cell for half its length, while the other 

 half agrees just as closely with a typical thick- walled prosenchymatous 

 wood-fibre. This is evidently a case, in which two physiological func- 

 tions find separate histological expression in two sharply differentiated 

 portions of the same cell. A further link in the chain is supplied by 

 those xylem-parenchyma cells which are distinctly thick-walled ; in 

 these, all the walls may be equally thick {Convolvulus Cneorum), or 

 thickening may be confined to the radial walls {e.g. the autumn-wood of 

 Magnolia acuminata, M. tripetala, Liriodendron tulipif&ra, Gymnocladus 

 canadensis, Amorphi fruticosa, according to De Bary). In the case of 

 these thick-walled parenchyma cells, as also in that of the intermediate 

 cells with narrow pits which occur in a great many plants, the mechanical 

 function is always of subordinate importance. These last-mentioned 

 cell-types, therefore, effect the final transition to normal thin-walled 

 xylem-parenchyma. 



The second series of transitional cell-forms may be tabulated as 

 follows : 



Cell Form. Principal Function. Subsidiary Function. 



1. Ordinary dead wood-fibres. Mechanical sup- 



port. 



2. Wood - fibres, with living Mechanical sup- Storage and con- 



contents port. duction of car- 



(a) Non-septate. bohydrates. 



(b) Septate. 



.3. Composite elements, which No differentiation of principal and 

 are wood-fibres as regards subsidiary functions, 



one half, and intermediate 

 cells as regards the other. 



