14 



OF WOOD IN GENEEAL 



be best understood from the diagrams (Fig. 9). In the thicken- 

 ing of the cell-wall the area of the outer circle is at first un- 

 thickened, but successive layers of thickening overlap this 

 unthiekened area more and more so as to make a short canal 

 broad at the end near the original cell-wall and narrow at the end 

 towards the centre of the cell. Subsequently a shght thickening 

 termed the torus forms m the centre of the unthiekened area. 

 Pressure of hquid on one side of the pit-membrane often forces it 

 against the " border," m which case the torus does not completely 

 occupy the opening in the border or inner circle. The whole 

 mechanism has been compared to a laboratory filter, the border 

 being the funnel that acts as a support, the unthiekened mem- 

 brane, which is permeable, corresponding to a filter-paper and the 

 torus to the small platinum cone sometimes placed in the middle 

 of the filter to protect it from direct pressure of liquid. The 

 bordered pits on xylem vessels in Oak have been compared to 



AJ 



AZ 









FiGr 9 —Pits A , Simple pit ; ^1, m tangential longitudinal section , A2,m surface 

 view B, Bordered pit , Bl, m tangential longitudinal section , £2, the same, with 

 the middle lamella thrust to one side ; Bd, in surface view ; J54, in semi-profile 



screw-heads, discs traversed by an elongated mark like the groove 

 for a screw-driver, and the structure has been explained by the 

 following imaginary model :^ " Imagine a pair of watch-glasses 

 each pierced by a narrow sht, and imagine them united face to face 

 with a dehcate circular piece of paper between them, and then fixed 

 into a hole cut in a thick piece of card. The outhne of the screw- 

 head is the outline of the umted watch-glasses where they are let 

 into the card ; the groove in the screw-head is the oblique cleft 

 which leads into the space between the glasses." In some cases, 

 under pressure from the cell-contents on the other side of it, the 

 unthiekened membrane in a pit bulges into the cavity of the 

 adjoining vessel. Such projections, which are known as tyloses, 

 may undergo cell-division and may even form a mass of tissue 

 blocking up the entire lumen of the vessel. This is the case in 

 some of the vessels of Oak and still more strikingly in the Locust 



^ Erancis Darwm, EUmefvts of Botany ^ pp 77-8. 



