SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 54I 



The phenomena of sclerosis now described appear in woody plants, to some 

 extent in immediate continuity with the primary differentiation of tissues, and 

 coincide with the first beginning of secondary growth in thickness; these cases, 

 especially the formation of sclerenchymatous rings at the limit of the external cortex, 

 could not, if they stood alone, be considered under the head of secondary formations. 

 Other phenomena, however, are so intimately connected with the former, that they 

 could scarcely be separated in the description, and these affect tissue-elements 

 which have often belonged for years to a definite differentiated tissue, namely, 

 the parenchyma, and only subsequently become involved in the sclerosis. The 

 first sclerenchymatous ring of the Beech, though it originates in the first period of 

 vegetation, increases every year in mass as it becomes shifted outwards during the 

 growth in thickness, owing to the fact that both those parenchymatous cells which 

 border on it externally, and others which are interpolated between its elements, in a 

 manner to be explained below, successively undergo sclerosis. The ridges of the 

 medullary rays arising from it constantly become broader, owing to the same 

 process, the further they are shifted outwards. According to Mohl and Sanio^, 

 the compound sclerenchymatous ring of Quercus Suber behaves in the same way. 

 In the secondary bast of the Beech, the White Fir, and other trees, sclerotic elements 

 are absent for at least 1-2 years. As each zone becomes shifted outwards, these 

 elements then appear in it in increasing numbers. The external cortex of the Beech, 

 the Horse-chestnut, and the White Fir is, in the first year, and often no doubt 

 during several years, free from sclerenchymatous elements, while in later years it is 

 abundantly permeated by them. Similar conditions prevail in the external cortex 

 and bast-layer of Drimys Winteri, and many other plants. 



So far as the existing data admit of a decision, the elements which are affected 

 by the secondary sclerosis always belong to the parenchyma, as was stated above ; 

 they may have discharged the functions of parenchymatous cells for years. Their 

 form and size appears in many cases not to be essentially changed on the occur- 

 rence of sclerosis (e. g. cortex of Fagus) ; in others a considerable alteration of 

 growth and form appears simultaneously with the commencement of this process. 

 This is most conspicuously the case with the many-armed, ramified, sclerenchyma- 

 tous elements of Abies pectinata, the arms of which are closely pectinated, forming 

 as it were a felt ; so far as is known, they originally proceed from polyhedral or 

 prismatic parenchymatous cells, both of the external cortex and of the secondary 

 bast. 



Sect. 172. Sieve-tubes, milk-tubes, sclerenchymatous elements, and sacs con- 

 taining crystals or other secretions, are, from their differentiation onwards, incapable 

 of further growth. They behave passively on dilatation, and become displaced from 

 their original position by the latter process. The longitudinal secretory canals also 

 share in the displacement, the tissue immediately surrounding them usually becoming 

 widened through the dilatation. In so far as the process of dilatation acts alone, the 

 displacement consists in a progressive lateral separation of the elements mentioned, 

 or of the strands into which they are united, and of the canals. If, as is often the 

 case, resistance is offered to the dilatation through the elasticity of the superficial 



1 Von Mohl, Verm. Schr. p. 220. — Sanio, Pringsheim's Jahrb. II. p. 73. 



