543 SECONDARY CHANGES. 



cortical layers for the time being, e. g. epidermis, external cortex and periderm, the 

 simple dilatational displacements are accompanied by others, the special forms of 

 which, varying greatly according to the special cases, need not be more minutely 

 described. 



Tlie resistent, sclerenchymatous elements, and also the crystal-sacs, do not 

 midergo any essential alteration of their structure during this process. Those 

 provided with soft walls, especially the sieve-tubes, and also many long secretory 

 sacs, suffer changes, simultaneously with the displacement, which, in general, consist 

 in the disappearance of the contents and collapse of the walls, and may be shortly 

 termed Obliteration. As this takes place under the joint action of the pressure 

 exercised in the radial and tangential directions, proceeding from the dilatation and 

 the resistance of the surface, it is obviously suggested to find in this the cause of the 

 obliteration. It is questionable; however, whether a change in the obliterating organs, 

 and especially in their contents, independent of the pressure, is not the primary cause 

 of the phenomenon, and the pressure only a co-operating cause. 



The obliterated sieve-tubes appear laterally compressed, even to the disappear- 

 ance of their lumen. Their structure, including that of the ends of their members 

 which bear the sieves, becomes indistinct, and may even be wholly unrecognisable ; 

 their walls appear slightly swollen, but no measurements have been made which 

 actually prove any swelling. Where the tubes are isolated, they are easily over- 

 looked after their collapse, seeming, at the first glance, to have quite disappeared. 

 Where they are united to form larger groups, their membranes appear collectively in 

 sections, especially transverse sections, as a homogeneous, gelatinous mass (like dry 

 gristle or horn), in which the compressed cavities are visible as narrow crooked 

 cracks or marks, the original lateral limits as indistinct lines. Similar appearances 

 have already been described on p. 325, and represented in Fig. 158, p. 336, in the 

 case of the primitive sieve-tubes of the vascular bundles. As the phenomenon 

 described often extends with apparent uniformity over the entire transverse section 

 of a considerable group of sieve-tubes, the question, how far the cambiform cells 

 which originally accompany the tubes, also share in the obliteration, remains to be 

 investigated. 



Obliterated groups of sieve-tubes have been described by Wigand ^ as ' horn- 

 bast,' while their origin and significance have been clearly represented by Rau- 

 wenhofif". 



The obliteration of the sieve-tubes begins in the oldest external zones of the 

 cortex, and advances, with the dilatation, in the centripetal direction. It seems to 

 come on more or less gradually or suddenly according to the particular case, a point 

 on which more minute investigations still remain to be undertaken. 



The obliteration of the secretory sacs has been described by Vogl '' in the case 

 of the large sacs of the Cinchona-bark. These apparently lose their original contents 



' Pringsheim's Jahrb. III. p. 118. 



^ Nederlandsch Kriudk.-Archief, V. p. 23. Compare also the treatise by the same author, Sur 

 les caracteres et la formation du liege, &c., reprinted in the Ann. Sci. Nat. 4 ser. torn. XV., and in 

 other places. 



' Die Chinarinden d. Wiener Grosshandels, p. 12. 



