SECONDARr CHANGES OUTSIDE THE ZONE OF THICKENING. 543 



and collapse, and the cavity thus formed is filled up by luxuriant growth of the 

 surrounding, dividing parenchymatous cells, until the wall of the sac may even 

 disappear. 



It has already been stated that those elements which are passive during dilata- 

 tion, remain in their original close connection one with another during the process of 

 separation, if they are united into strands. If, on the other hand, as is in fact often 

 the case with the sclerenchyma, they form closed annular layers, the latter are burst. 

 The severance of connection then takes place through the limiting surfaces of the 

 elements. As soon as this begins at any point of the external surface of a scleren- 

 chymatous ring, neighbouring parenchymatous cells bulge out and insert themselves 

 into the gap, which they fill up. They then either maintain the properties of 

 parenchymatous cells, growing and dividing in proportion to the increasing width of 

 the gap owing to progressive growth in thickness, or they undergo the sclerenchy- 

 matous metamorphosis immediately upon their insertion ; the original ring becomes 

 completed by interpolated short stone-elements, as described above. 



The latter process occurs in the stony or compound sclerenchymatous rings, 

 which, e. g. in the Beech, always remain closed without any growth of the existing 

 sclerotic elements, while their circumference constantly increases. The former case 

 may occur in the same rings, and takes place especially in rings of fibrous scleren- 

 chyma, where the latter are not thrown off at an early period by internal formation of 

 periderm, as is usually the case ; e. g. the bulky fibrous ring of Aristolochia Sipho, 

 thick foliage-stems of Gypsophila altissima, and, in a lesser degree, in old thick stems 

 of the Gourd. In the above-mentioned Aristolochia especially, the ring is first burst at 

 the places lying opposite the bands of greatest dilatation, and then at a constantly 

 increasing number of other points, and is thus broken up into segments, which are 

 always becoming smaller, and finally often consist only of single separated fibres, 

 the space between them being filled up by thin-walled parenchyma, which follows the 

 dilatation. In this case also, however, a partial completion of the ring by short 

 stone-sclerenchyma may take place, owing to secondary sclerosis. 



Sect. 173. The generally distributed phenomena of displacement and obliteration 

 may be accompanied in special cases by processes of disorganisation, and the latter 

 may extend to tissues of every kind. In the cortex of the Amygdalese, e.g. Prunus avium, 

 groups of tissue of varying extent become disorganised, and converted into cavities 

 filled with gum and bassorin, from which the swelling contents, consisting of cherry- 

 gum, finally exude through the bursting surface of the cortex. According to Wigand's 

 statement ', it is chiefly the obliterated sieve-tubes in the cortex from which this disor- 

 ganisation starts, and then further extends throughout the non-equivalent tissues. On 

 the other hand, the latter also, especially groups of thick- walled parenchymatous cells 

 which have been called abnormal, are certainly starting-points of the gummy dis- 

 organisation. On the very various special phenomena, which may, for the most part, 

 belong to the province of Pathology, comp. Wigand's description. 



In the older cortex of many Coniferse, besides the protogenetic reservoirs of resin 

 (p. 441), and to some extent as a substitute for the latter when they have been lost 

 owing to formation of bark, reservoirs filled with balsam appear, which were termed 



• /,c. p. 130. 



