470 SECONDARY CHANGES. 



tripetala^ The above-mentioned elongated elements of the Dicotyledonous wood 

 preserve their radial arrangement, for example '^ in Cunonia capensis, Viburnum 

 Opulus, Staphylea, Hamamelis, Nerium, many Asclepiadeae, Rhus typhinum, Jatropha' 

 Manihot, Laurus nobilis, Camphora, Aesculus, Verbena maritima, Broussonetia, 

 Catalpa, Paulownia, Hydrangea hortensis, Justicia carnea, Fuchsia, Melastomacese', 

 &c., which are chiefly, though not exclusively, plants with leaves in whorls ; not but 

 what with similar phyllotaxis a different arrangement of the elements may occur, as 

 will be shown by facts to be mentioned immediately. 



The original radial arrangement is, on the other hand, disturbed or obliterated : 



(i) In groups of elongated elements, which show a great elongation on transition 

 from the cambial condition to that of mature tissue, in the course of which they insert 

 their tapering ends, which are the principal seat of growth, between each other, and 

 acquire terminal surfaces which are inclined not only towards the radial rows, but also 

 in other directions, or even show curvature and torsion of their ends. 



(2) When certain of the originally similar elements undergo considerable growth 

 in the transverse directions, on attaining their definitive development. 



The first case perhaps occurs during the formation of many irregular groups of 

 sieve-tubes, but this has not been minutely investigated, and is doubtful. It certainly 

 takes place, however, in the case of those fibrous elements, which in the mature 

 condition do not show any regular serial arrangement, and which often grow to 

 many times their original cambial length : as in the groups of sclerenchymatous 

 fibres in the bast of many Dicotyledons, e.g. Tilia (Fig. 211), in the fibrous cells, 

 fibres, and elongated tracheides in the wood of Leguminosse (Cytisns Laburnum, 

 Caragana, &c. ; most beautifully, on account of the contrast with the other elements 

 of the wood, which maintain the cambial form and length, in Herminiera Ela- 

 phroxylon, comp. Sect. 150), Ulmus suberosa, Morus alba, Celtis australis, Tamarix 

 gallica. Ilex aquifolium, Cornus sanguinea, Pyrus, &c. 



The second case occurs universally in the development of wide vessels. Ori- 

 ginally similar to the other elements, the members of the vessel often become 

 expanded to many times their initial size ; the neighbouring elements thus become 

 not only displaced, but often transversely deformed in the direction of the surface of 

 the vessel, compressed, or even completely crushed, so that mere rudiments remain '. 

 According to the degree of expansion, and the number of the wide vessels in any 

 portion of the transverse section, the general arrangement of the elements is 

 influenced by them. 



It need scarcely be mentioned that all the phenomena described may occur in 

 various degrees, so that cases i^ntermediate between the extremes may exist. 



Sect. 138. The collective zones of secondary growth, cambium, young and 

 mature wood, bast, &c., which have hitherto been regarded with immediate reference 

 to a single transverse section of stem and root, if traced longitudinally, are continued, 

 as uninterrupted layers, both through the successive portions of the same axis, and 



' Hartig, Forstl. Culturpfl. p. 256.— Sanio, Botan. Zeitg. 1863, p. 107. 



^ Compare Sanio, I.e. pp. 107, 115. 



' Vochting, /. c. 



* For a minute description, see Velten, Botan. Zeitg. 1875, p. 809, &c. 



