bG4 



SECONDARY GROWTH IN THICKNESS 



which link up typical wood-fibres, on the one hand, with vessels 

 and tracheides on the other. The first indication which suggests that 

 a wood-fibre has undertaken the transportation of water as a subsidiary 

 function, consists in the appearance of small bordered pits on its 

 walls ; the cavities of these pits are still narrow and oblique (sinistrorse), 

 but the enlargement of the limiting membranes is sufficient to produce 

 a small chamber (Quercus, Fraximcs, Da/phne, etc.). As the water- 

 conducting activity of the wood-fibres becomes more pronounced, their 

 bordered pits become larger and more typical, until finally the cells are 

 almost equally adapted for performing mechanical and conducting 

 functions. These strictly intermediate cells are prosenchymatous and 

 thick-walled, and bear large circular bordered pits, with rounded, or with 

 narrow oblique orifices. Cells of this type, which may be termed 

 fibrous tracheides (Fasertracheiden), are typically developed in the Mag- 

 noliaceous genus Drimys, and even more so in the wood of Conifers. A 

 still closer approximation to the structure of typical water-conducting 

 elements is found in Taxus, and in other cases where the fibrous 

 tracheides are furnished with spiral thickenings. From this condition 

 it is only a small step to those thick-walled vessels and tracheides the 

 mechanical function of which is purely subsidiary (Fraxinus excelsior, 

 JVerium Oleander, Convolvulus Cneorum, etc.). Thin-walled vessels and 

 tracheides represent the final link in the series. 



For the sake of clearness, the above-mentioned transitional forms 

 may be tabulated, with reference to their respective functions, as 

 follows : 



4. 



Cell Form. 



Wood-fibres with narrow 

 simple pits. 



Wood - fibres with small 

 bordered pits (narrow pit- 

 cavities). 



Fibrous tracheides. Prosen- 

 chymatous, thick- walled 

 cells, with large bordered 

 pits. 



Fibrous tracheides with spiral 

 thickenings, otherwise as 

 in 3. 



Thick-walled 

 tracheides. 



Thin -walled 

 tracheides. 



Principal Function. Subsidiary Function. 



Mechanical sup- 

 port. 



Mechanical sup- Water-conduction, 

 port. 



No differentiation of principal and 

 subsidiary functions ; the cells 

 serve equally for mechanical sup- 

 port and for water-conduction. 



vessels and 



,-i'S.si'h 



and 



Water- conduc- 

 tion. 



Water -conduc- 

 tion. 



Mechanical sup- 

 port. 



The second set of intermediate cell-forms effects the transition from 

 typical wood-fibres to thin-walled xylem-parenchyma. This series is 



