SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 535 



the remarks and citations upon p. 500 ; on the processes of degradation in wood 

 when ageing, the relation of which to these phenomena is, to say the least, very 

 doubtful, see Sect. 154. The primary medullary bundles of the forms here in 

 question — Piperaceae, Begonise, Aralise and Umbelliferse, Mamillarise, and Melasto- 

 maceae (comp. Sect. 62) — undergo, so far as is known, no secondary anatomical 

 changes. 



Sect. 169. All the parts lying outside the active cambial zone, namely, the 

 entire primary cortex and ihe secondary cortex for the time being, necessarily undergo 

 progressive changes with the progressive growth in thickness. These consist in — 



{a) Growth of existing tissue-elements, new formation of equivalent ones from 

 them, and subsequent metamorphosis (p. 5): Sects. 170, 171. 



((5) Compression, displacement, and destruction of existing tissues: Sects. 172, 



173- 



(<r) New formation of non-equivalent forms of tissue out of existing ones : 



Periderm, Sects. 174-179. 



The process indicated by {a) only affects the cellular tissues : Epidermis and 

 Parenchyma. 



Sect. 170. In the majority of the cases of vigorous growth in thickness, the 

 epidermis is destroyed at an early period, cork or bark being formed ; further details 

 will be given below. Stems, however, are not wanting, whether with weak or with 

 very vigorous growth in thickness, in which the epidermis follows the latter by its 

 own growth during a considerable period. This is the case in many herbaceous 

 plants, and in woody plants with a smooth, green, cortical surface, as long as the 

 latter is present. The condition of the surface mentioned is in fact dependent on 

 the persistency of the epidermis, the cells of which, being filled with sap, allow the 

 colour of the sub-epidermal chlorophyll to show through. As examples of stems and 

 branches with vigorous secondary growth of the wood, which, for some years at least, 

 retain the epidermis, may be mentioned : Viscum album ^, species of Ilex, the ever- 

 green Jasmines, Menispermum canadense, Aristolochia Sipho and allies, Sophora 

 janonica, Negundo, and many others. In Acer striatum the epidermis is still for the 

 most part present in a living condition, and following the growth, even on stems a 

 foot thick, and forty years or more old. 



The long-lived epidermis of the woody plants mentioned is provided from the 

 first with thick strongly cuticularised outer w^alls, which sometimes contain and 

 excrete a large amount of wax (comp. pp. 76, 82). Its original structure under- 

 goes relatively unimportant changes during growth. These consist in an increased 

 thickening of the cuticularised external walls, the surface of which covered by the 

 cuticle usually remains smooth ; but in Acer striatum as well as in Negundo and So- 

 phora japonica it becomes cracked as the thickening proceeds, the cracks penetrating 

 from outside into the external cuticular layers, which do not follow the growth, and 

 successively breaking them up into crumbling fragments. In Acer striatum the 

 cracks coincide with the dilated bands of the .external cortex, to be described below ; 

 and a new excretion of wax rods takes place in each case on the surfaces newly 

 laid hare hy tlie cracks ; it is on this that the white striation of the cortical surface 



^ Von Mohl, Bolan. Zeitg. 1859, p. 593. • 



