544 



SECONDARY CHANGES. 



by Mohl ' resin-cavities ; their origin is no doubt always lysigenetic, resulting from 

 a disorganisation of definite groups of tissue. Among the Abietinese investigated by 

 Mohl their formation is wholly absent in several species, namely, Pinus sylvestris and 

 nigricans, Abies excelsa and pectinata ; they appear in Larix europsa, Abies sibirica, 

 and Pinus Strobus ; in the first-mentioned tree in the first year, in the two others not 

 before the eighth or tenth year of life, in the cases investigated. The seat of their 

 occurrence is' especially, and in Larix and Abies sibirica exclusively, the external 

 cortex, in P. Strobus the bast. also. As years go on they increase in number, and 

 the pre-existing ones in size; the transverse diameter is, for example, stated by 

 Mohl, in the case of Larix europsea, at little more than j-V°"" i" ^ young cavity one 

 year old, and at almost imm in a cavity eighteen years old. According to Mohl, the 

 form of these cavities in the trees mentioned is at first approximately spherical, and 

 afterwards passes over into a lenticular, transversely elongated form. Their origin 

 and enlargement by the solution of definite groups of tissue has not indeed been 

 described in detail, but scarcely admits of doubt, especially after Wigand's state- 

 ment ^ according to which they arise in the bast of Pinus Strobus from the solution 

 of groups of tissue, which contain obliterated sieve-tubes, parenchyma, and stone- 

 sclerenchyma. 



The mode of origin stated by Wigand to occur in Pinus Strobus, doubtless 

 applies to the cavities filled with resin, which appear in the form of longitudinal 

 canals, ending, so far as can be asserted, blindly, in the older bast of the Cupressine» 

 (Juniperus communis, Thuja, Biota, and Cupressus spec, comp. p. 443). Their 

 formation begins here ', at points in the older zones of bast which cannot be more 

 exactly indicated, while the latter are still turgescent, and still contain non-obliterated 

 sieve-tubes ; resin first makes its appearance in isolated, otherwise unaltered, parenchy- 

 matous cells, both in the bundles and in the medullary rays ; the resiniferous cavity 

 then arises by means of an increase in the amount of resin, and successive solution 

 of the membranes, which perhaps themselves afford material for the formation of 

 resin ; this cavity becomes widened by the extension of the same process within a 

 constantly increasing circuit, and over all the surrounding tissue-elements of the 

 bast, sieve-tubes and fibres not excepted. In the parenchymatous cells bordering 

 the passage, a considerable enlargement in the radial direction occurs in the case of 

 Juniperus, with papillose protrusions towards the passage, while isolated divisions by 

 tangential walls are also frequent. 



Periderm *. 



Sect. 174. In addition to the changes in the growing cortical mantle already 

 described, more profound ones ensue on the new formation of the phellogenetic, 

 i. e. cork-producing merisfem, and its products. It may be expedient to include all 



' Botan. Zeitg. 1859, p. 333. 2 /. c. p. 166. » Frank, Beitr. p. 122. 



' Von Mohl, Unters. iib. d. Entwickelung des Korkes nnd der Borke auf der Rinde der bau- 

 martigen Dicotylen; Diss. 1836.— Verm. Schriften, p. 212.— Hanstein, Unters. ub. d. Bau u. d.Entw. 

 d. Baumrinde; Berlin, 1853.— Sanio, Unters. iib. d. Bau u. d. Entw. d. Korkes, Pringsheim's Jahrb. 

 II. p. 39. 



