35] CHAPTER II. THE TISSUES. 147 



sively of tracheids ; but where some of the elements (as generally 

 in the wood of Dicotyledons) attain a much greater size (as seen 

 in transverse section, Fig. 105), the original radial arrangement 

 is lost. 



In those cases in which the permanent tissues consist of very 

 long or very wide fibres or vessels, it is evident that the relative 

 position of the original cells must have undergone considerable 

 change in the course of development ; the long fibre is in contact, 

 longitudinally, with a greater number of cells than was originally 

 the case ; and similarly, the wide trachea touches, at its circum- 

 ference, a larger number of cells than did the cell, originally, from 

 which the segment of the vessel was developed. This gradual 

 change of relative position constitutes what is termed sliding- 

 groicth ; it is the expression of the independent growth of each 

 cell in the course of its development into the particular element 

 of the permanent tissue which it is destined to form. This process 

 is by no means confined to the vascular tissues, but takes place 

 wherever a young developing cell grows more actively, in any 

 dimension, than the cells with which it is at first in contact ; a 

 notable example is the growth of the laticiferous coenocytes of 

 Euphorbia (see p. 100). 



Whilst undergoing these changes of form, the developing cells 

 undergo, as already indicated, changes in the structure and 

 chemical composition of their cell-walls in accordance with the 

 particular kind of tissue to which they are to give rise ; and, in 

 some cases (tracheae, tracheids, fibres) they lose their protoplas- 

 mic cell-contents ; the walls become more or less thickened, not 

 spiral or annular as in primary wood, but pitted (with simple 

 pits ; or circular bordered pits ; or oval bordered pits, either small 

 and numerous, or large extending across a whole face of the wall, 

 giving it a scalariform appearance, see p. 74) ; and then the 

 absorption, more or less complete, of the septa takes place, which 

 leads to the formation of the vessels. 



Glandular tissue is frequently developed in the secondary wood and 

 bast, in the form, sometimes, of sacs containing crystals, in the paren- 

 chyma (including medullary rays) of the wood (e.g. Vitis, and some 

 leguminous trees) or more commonly in that of the bast : of resin-ducts 

 which occur in the secondary wood of certain Abietineae, running hori- 

 zontally in the medullary rays and vertically in the wood, but rarely 

 found in the secondary bast, whereas in other plants which possess these 

 structures, they are rare in the wood but abundant in the bast (e.g. 

 Anacardiaceae, etc.) : of laticiferous vessels, rare in the wood (except the 



