PITS. 11 



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 

 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. 1 " Imagine a pair of watch-glasses 

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

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

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

 head is the outline of the united 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 

 unthickened 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 

 or Acacia (Robinia Pseudacdcia), in which the wood consequently 

 appears non-porous, but, their cell-walls being thin, the tyloses 

 appear in transverse section as light yellow spots on the dark 

 heartwood. In Letterwood (Brdsimum AubUtii), on the other 

 hand, the tracheae are filled up with tyloses, the cells of which 

 have their walls very much thickened so that they appear dark. 



We come next to the tissues which are of the greatest im- 

 portance in our present study those of the xylem or wood, 

 developed on the inner side of the procambium strand and 

 subsequently on the inner side of the cambium sheath. The 

 development of xylem in a procambium strand begins with the 

 conversion of one or a few cells, or vertical rows of cells, of 

 the inner part of the strand into spirally, or occasionally 

 annularly, thickened tracheids or trachea, known as the protoxylem 

 or first-formed wood. This conversion consists in the loss of 

 their protoplasmic contents, the lignification of their walls, the 



1 Francis Darwin, Etemtnte of Botany, pp. 77-8. 

 B 



