SECONDARY XYLEM 669 



2. The arrangement of tissues in the woody cylinder. 



In considering the distribution and arrangement of the various 

 elements of which the woody cylinder is composed, we are confronted 

 with two questions ; we have to decide, first, how far the arrangement 

 is merely a geometrical or mechanical consequence of the process of 

 secondary growth and its attendant changes ; and, secondly, to what 

 extent the arrangement depends upon, or is determined by, the functions 

 of the elements concerned. The former of these two closely related 

 questions may be dealt with first. 



In the secondary wood, as in the secondary phloem, the newly 

 differentiated elements are arranged in radial rows. If the conversion of 

 these cells into permanent tissue is only attended by radial elongation, 

 while their tangential dimensions undergo no change, the radial seriation 

 naturally remains unaltered. This condition is exemplified by all 

 ordinary xylem-parenchyma at least when it forms large continuous 

 masses by the medullary rays and by tracheidal tissue. The radial 

 seriation will likewise be maintained, if the ends of the elongating cells 

 are all inclined in tangential planes, so that the cells can slide past one 

 another without lateral deviation. It is for this reason that the fibrous 

 tracheides of Conifers are always disposed in perfectly regular radial 

 rows ; among Dicotyledons the wood-fibres very frequently exhibit a 

 similar arrangement (e.g. in Viburnum Opulus, Nerium Oleander, Laurus 

 nobilis, Aesculus Hippocastanum, Paulownia imperialis, etc.). In other 

 cases the original radial seriation undergoes considerable modification, 

 or even disappears altogether. This alteration may be brought about 

 by the expansion of individual cells during their conversion into per- 

 manent elements ; every large vessel, for example, displaces, compresses 

 or even obliterates the cells in its immediate neighbourhood. The 

 disturbance may also be due to the circumstance that the elongating 

 cells become dovetailed together in various ways ; this latter arrange- 

 ment is exemplified by many wood-fibres (e.g. those of Ulmus suberosa, 

 Morus alba, Celtis australis, Cornus sanguinea, Gytisus Laburnum and 

 other Leguminosae, etc.). In either case a considerable amount of 

 sliding growth must take place. 



The morphological relations that have just been explained, are of less 

 interest than the connection which exists, between the arrangement of 

 the various constituents of the secondary wood, on the one hand, and 

 their several physiological functions, on the other. In considering this 

 second question, it must always be borne in mind that the woody 

 cylinder is made up of elements belonging to two distinct tissue- 

 systems. Obviously, therefore, any given arrangement of the com- 

 ponent elements of the secondary xylem will not be satisfactory, 



