196 SECTIONAL ADDRESSES ' 



has not shifted upwards or downwards as a whole. When the file of 

 tracheids doubles, through the transverse division in the cambium 

 initial, the new files of tracheids are at first nearly superimposed vertically, 

 but as the two initials grow in length, the cross wall between them 

 becomes more tilted, especially in the tangential direction, so that the two 

 files of tracheids cut off from them come to lie more nearly side by side. 

 The readjustments in relative position of the two daughter initials, on 

 the face of the files of tracheids, may take place without slip, in view of the 

 liquid nature of the cell contents and the plastic nature of their walls, 

 which allows of a relatively rapid extension of the new ' three-ply ' 

 cellulose-pectin-cellulose division wall as it moves into a more vertical 

 position. 



Reasons have been given elsewhere for thinking that these relatively 

 minor readjustments of position in the cambial cells, as the cambium 

 cylinder grows wider, thus take place by ' symplastic ' movement of the 

 common framework of walls of the fusiform initials, which thus continue 

 to grow and spread tangentially between the rays. The latter act as fixed 

 points because they are formed of vacuolated cells which extend through 

 to the differentiated vascular tissues on either side. When grovvth in the 

 cambium is renewed in the spring, if the impetus to grovvth and tangential 

 expansion reaches one point first at any level in the cambium cylinder 

 and spreads from thence gradually round the cylinder, this plastic wall 

 framework of the initials will tend to undergo an oblique displacement 

 as a whole. The result, when the new pattern is repeated in the wood, 

 will be the production of ' twisted fibre ' or ' spiral grain.' This is a 

 phenomenon that frequently thrusts itself upon the attention of the 

 forester and worker in wood. 



In the early summer quite a number of tracheids in the same radial 

 file are in course of differentiation, and the radial expansion of these 

 vacuolating elements must exert a very considerable pressure upon the 

 plastic meristematic cambium initials outside them, which thus remain 

 radially compressed and seem to become even more elongated in the 

 young tree each year as the vigour of radial growth increases. This radial 

 pressure was very vividly brought home to us when we were stripping the 

 young differentiating tissue off the old wood. When a thick layer of 

 differentiating tracheids was present, as the knife scraped these tissues a 

 fine spray rose from their surface to a height of more than a foot. This 

 phenomenon has only been noticed with the softwoods, and supplies the 

 clearest evidence of the pressure under which the liquid contents may be 

 held in the vacuolating tissues. 



Cambium and Vascular Dijferentiation in the Hardwood. — Bailey has 

 pointed out that the cambia of the Dicotyledons may be grouped in 

 different categories in which, as the initial becomes less elongated, the 

 specialised development of wide vessel segments and elongated fibres 

 becomes more pronounced. The shorter cambium initial shows an 

 obvious correlation with the broader leaf primordium and diminished 

 longitudinal extension of the original meristem cells. In very short 

 cambial cells division walls, which are originally nearly transverse, will 

 not be pulled out into an oblique or nearly vertical position by the sub- 



