[anomalous thickening in dicottledons and gtmnosperms. 605 



a less extent at the margins of the plate than in the middle. This explanation, however, 

 is not enough, since the successive elongation of the middle is greatest in the oldest 

 zone surrounding the pith. The same holds as regards the attempted explanation made 

 by Netto\ If the assumption of a shortening of the margins of the plate be excluded, 

 the other alone remains, that in the old zones there is a continued growth in length ; 

 and since according to other data, known in the case of Angiospermous trees, such a 

 growth in other elements, though not impossible, is less probable, while phenomena of 

 growth may be proved to occur elsewhere in the older parenchyma, the anatomical 

 basis of the elongation must first be sought for in the latter. For the proof of 

 this assumption, and the demonstration of the anatomical changes which necessarily 

 follow from it in the old zones, more accurate investigation is wanted, and especially 

 measurements. 



In old stems the swollen projections of the xylem increase in size, and the islands 

 of parenchyma in width. The final splitting of the former into the pieces and lobes 

 (Fig. 238) with independent growth can hardly come about otherwise than as above 

 stated, but this point requires more exact investigation. The anomalies of structure are 

 continued, according to Cruger, into the roots. 



In the stems under consideration the whole parenchymatous tissue surrounding 

 the isolated segments of wood, together with the periderm, follow their unequal growth 

 for a long time by means of corresponding dilatation. 



Especially in the Malpighiacese and the Bauhinise, however, the inequalities of 

 growth, as the increase of thickness proceeds, are so great that the masses of tissue lying 

 between the ligneous bodies are ruptured. At the surfaces of separation a formation 

 of periderm takes place; perhaps also a throwing off of single bands of tissue 

 as bark; this however has not been closely investigated. The stem now appears 

 split into more or less numerous longitudinal strands, each of which is covered by 

 a special cortex, coated with periderm; these are arranged in the most various 

 manner parallel to one another, or are interwoven one with another, and separated 

 for a distance and again united. 



The above-mentioned behaviour of the stem oHUrvillea laevis^ belongs to this cate - 

 gory, because in the second or third year the wood begins, as in the Malpighiaceae, to 

 become three-lobed. ' By growth in width of the parenchyma of the medullary rays 

 the three segments of the wood corresponding to the three lobes are separated from 

 one another, and each of them, with the adjoining third of the pith, is surrounded by 

 a zone of cambium which thenceforth forms wood and bast, so that the stem contains 

 three separately growing masses of wood and bast. 



As less striking and fundamental internal secondary formations we may connect 

 with the above those strands of wood- which arise subsequently from secondary 

 meristem, as described by Criiger, /. c, in the pith of Doliocarpus, Argyreia, &c. ; it is 

 doubtful whether the masses of wood, which appear lobed in transverse section, 

 and lie, as described by Oliver ^, enclosed in parenchyma in the stem of Acantho- 

 phyllum (of the family Caryophyllaceae), belong to this category. 



The formation of bast on the inner surface of old hollow stems of Carica Papaya 

 should rather be included in the province of phenomena of pathological callus 

 formation and regeneration. 



' Ann. sci. n^t. 5 Ser. torn. VI. p. 317, &c. 



2 Radlkofer, Atti, /. c, p. 63.— Gaudichaud, I.e., Tab. XVIII. Fig. 20. 



* Trans. Linn. Soc. London, Vol. XXII, p. 289. 



