SECONDARY THICKENING. NORMAL DICOTYLEDONS. 505 



true woody plants. The accurate distinction of the individual elements one from 

 another therefore presents great technical difficulties ; even in good transverse sections 

 the recognition of the original xylem-plates between their thick-walled next neighbours 

 is often very difficult, besides which the narrow vessels at their angles often seem to 

 become indistinct owing to pressure from the surrounding tissues. 



In addition to these peculiarities of the internal limit of the wood in stem and 

 root, further peculiarities exist in the inner secondary ligneous mass itself. 



In rare cases, especially in the stem of Mahonia Aquifolium, Berberis vulgaris, 

 Pelargonium roseum, and Solanum Dulcamara, Sanio found septate fibrous cells in 

 the first annual ring, while they are absent in the succeeding ones. And further, in 

 the first and next-following annual rings of the stem and its branches, in many 

 though not all Dicotyledonous woods, although the elements characteristic of the 

 species are all present, yet their characteristic arrangement does not appear clearly till 

 later, as it is merely indicated in the former region. The groups of vessels, and the 

 parenchymatous zones of Hedera Helix, Quercus pedunculata, Juglans, Casuarina, &c.', 

 are examples of this. The characteristic structure of a wood is therefore not always 

 to be recognised with clearness from its inner rings. Similar phenomena, which have 

 not at present been accurately investigated, may occur in the case of roots. 



Sect. 153. The most remarkable phenomenon of this category is the change in 

 the average size of equivalent elementary organs, as regards both their width and 

 length, which accompanies the growth in thickness. In most woods this change 

 takes place in such a manner that the average size increases during a series of years, 

 and then attains a definite value which remains constant in the succeeding years. 

 The change in question takes place in a different manner in the stem, branches, and 

 roots of the same tree, and in their different transverse zones; in some cases it 

 depends on a corresponding successive increase in size of the cambial cells, while in 

 others it is independent of this. 



The most complete measurements referring to this point were carried out 

 on Pinus silvestris by Sanio '\ who determined the mean length of the tracheides, 

 and their mean tangential breadth in the autumn wood. He thus sums up his 

 results : — 



1. in the stem and branches the tracheides everywhere increase from within 

 outwards, throughout a number of annual rings, until they have attained a definite 

 size, which then remains constant for the following annual rings. 



2. The constant final size changes in the stem in such a manner that it con- 

 stantly increases from below upwards, reaches its maximum at a definite height, and 

 then again diminishes towards the summit. 



3. The final size of the tracheides in the branches is less than in the stem, but 

 is dependent on the latter, inasmuch as those branches which arise from the stem at 

 a level where the tracheides are larger, themselves have larger tracheides than those 

 which arise at a level where the constant size is less. 



4. In the gnarled branches of the summit, the constant size in the outer annual 

 ring also at first increases towards the apex, and then falls again ; but here irregu- 

 larities occur, which may be absent in regularly grown branches. 



' Sanio, Botan. Zeitg. 1863, p. 397. * Pringsheim's Jahrb. VIII. p. 401, &c. 



