K.— BOTANY 199 



the fibre as the natural corollary of the rapid basipetal spread of this 

 tendency to expand in the files of elements which will form the 

 vessels. The fibres are often compressed to a length several times that 

 of the original cambium initials. It is concluded, from the evidence as 

 to correspondence of pits, that these future fibre elements deform under 

 compression in a symplastic manner, their walls changing position as 

 a common framework. As the cambial activity begins in the leaf trace 

 and proceeds thence basipetally downwards, vessel segments are also 

 added in succession basipetally and the compression of the tissue between 

 the vacuolated vessels and rays also takes place in the same basipetal 

 sequence. Thus a wedge of expanding differentiating tissue spreads 

 downwards, on the surface of the old wood, inside the growing and 

 dividing cambium, which is carried outwards upon this living and ex- 

 panding framework without undergoing much direct radial pressure and 

 without an appreciable increase, as a rule, in the length of the individual 

 cambial initials. 



In this manner the surface of the wood is clothed throughout its 

 length with a new layer of wood, which originates and spreads from the 

 base of the extending foliage shoots, and which consists of thin walled 

 wood elements in which vessels are relatively numerous. But when the 

 leaves are fully expanded and are busily engaged in photosynthesis, a good 

 proportion of the new tissue formed from the cambium is phloem, whilst 

 the new wood elements have much thicker walls and the vessels are not 

 so prominent. This rhythm of structural differentiation builds up the 

 annual ring which, in temperate climes, where foliar expansion all takes 

 place at a definite season, forms such a characteristic feature of the annual 

 increment of wood. This growth and differentiation of new secondary 

 tissues, in such close connection with leaf expansion and physiological 

 activity, cannot fail to have a great influence upon the growth and func- 

 tional activity of the new leaf system. Curiously enough, although the 

 attention of many observers has been attracted by the problems presented 

 by the maintenance of the water supply to the leaves at the top of a tall 

 tree, and by the removal of the products of photosynthesis from the 

 leaves for the nourishment of the rest of the tree, including the root 

 system, these problems are usually considered as if detached from the 

 phenomena of growth in the tree. We have noticed already that if the 

 buds on the lower branches fail to grow, cambial activity also fails in 

 these branches, which thereupon dry, losing their water, if not their food 

 supplies, to the regions in the tree which are still growing. It is clear 

 that any interpretation of water and food movement in the tree must be 

 inadequate which neglects to consider these problems in relation to 

 growth and differentiation, and these problems will now be briefly 

 reconsidered from this standpoint. 



III. Function. The Movement of Water and Solutes in 



THE Tree. 



Water Movement at Bud-break.— The opening of the buds in spring 

 must be associated with the movement of water into their tissues. In 



