Notice of Book. 131 
the radial direction, as is actually the case during the development of 
the young wood from the cambium. Owing to this radial extension 
of the tissue the growth of the vessel is able to take place without 
involving the obliteration or even any serious reduction in the 
dimensions of the neighbouring cells. In this case it is only the 
tangential extension of the vessel which is due to sliding growth, the 
increase in its radial diameter being accounted for by the growth of 
the tissue as a whole. This, however, only applies to vessels of 
moderate size ; the very large vessels, such as are found in the spring 
wood of the oak, require, as we shall see, sliding growth in the radial 
direction also for their development. 
In connection with this part of the subject the author discusses the 
interesting question, whether the extension of the vessel is due to 
growth all round or to localised growth at the points where it pene- 
trates between the neighbouring cells. He decides in favour of the 
latter alternative. There is nothing improbable in this view, as 
localised growth often occurs in other cases ; at the same time this 
conclusion cannot be said to rest on any decisive observations. 
In the formation of the larger vessels, as already mentioned, sliding 
growth must take place in the radial, as well as in the tangential 
direction, for here the radial extension of the vessel is in excess of 
that of the young wood generally. It is obvious that in this case the 
growth of the vessels inwards, that is, towards the already formed 
wood, can only go on so long as the cells in this region are still 
capable of extension. Towards the cambium the radial growth will 
be able to go on for a longer time. 
Dr. Krabbe next proves that the growth of the vessel may induce 
sliding between cells not immediately in contact with it. This will be 
the case wherever the radial growth of the vessel is greater than the 
average radial growth of the young wood, for here the growth of the 
cells in the same radial row with the vessel will be less than the 
average, as the cambium is not displaced. Hence sliding must take 
place between the slowly growing cells of this radial row (which are 
passively pushed out by the growth of the vessel) and the cells of the 
neighbouring rows, which grow at the average rate. 
Another case of induced sliding growth occurs wherever the vessel, 
in its tangential extension, exerts pressure on a medullary ray. The 
medullary rays are never interrupted by the growth of the vessel, a 
point of some physiological interest ; but they are often, as it were, 
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