478 SECONBARF CHANGES. 



annual secondary growth from below upwards must be different from that in our 

 trees, in so far as the form is dependent on the thickness of the layers of wood, 

 and not on that of the cortical layers or masses of pith. How far the one or the 

 other explanation is the right one has not been made out in all cases in these plants. 

 The Mamillariee, for example, cited by Mohl as examples of barrel-shaped stems, 

 owe their form, not to the successive increase or decrease in thickness of the layers 

 of wood, but to that of the cortical masses of parenchyma. 



2. TAe forms of Tissue of the secondary wood. 



Sect. 141. f\is forms of tissue of which the secondary wood is composed' 

 belong chiefly to the categories oi cells (comp. pp. 5, 115, 121), trachea, and scleren- 

 chymatous elements, especially sclerenchymatous fibres. They sometimes have their 

 characteristic anatomical peculiarities, and the division of labour which these in- 

 dicate, rigorously developed and carried out; sometimes, however, the division of 

 labour is arranged in such a manner that while an element has the essential pecu- 

 liarities and functions of one of these forms of tissue, and must therefore be classified 

 with it, it further shares in the characteristics of another form. 



In the tough strong woods of trees and shrubs, which have been the chief 

 subject of investigation, the latter holds good of the elements of all forms, in so far 

 that they are — in various degrees — thick-walled and sclerotic ; a phenomenon which 

 is not characteristic of the secondary wood generally, but only of that of the ' woods ' 

 so called in ordinary phraseology. In the very hard secondary wood of Convolvulus 

 Cneorum, for example, all the elements both of the bundles and of the medullary rays 

 are in the highest degree sclerotic; in the soft, fleshy, chiefly parenchymatous wood 

 of the stem of Carica, Cheirostejnon, many succulent roots, &c., only certain individual 

 elements have that character. In the stem of Clematis Vitalba, the parenchyma of 

 the medullary rays takes part in the sclerosis, in that of Atragene it does not, 

 and so on. 



In many woods one or the other form of tissue may be absent, and its functions 

 be undertaken by others, as will be shown by the examples to be mentioned below. 



Sect. 142. The trachea of the secondary wood appear partly in the form of 

 vessels, partly as tracheides ^. 



Of the forms of vessels, as distinguished by the structure of their walls, the 

 reticulately thickened are present exclusively or principally in succulent soft woods, 

 as in the stem of the Papayaceae, and in many fleshy roots (Sect. 159). Reticulated 

 vessels with large meshes are further characteristic of the wood of the Crassulaceae 'j 

 even of the species with hard wood. Reticulated vessels occur, together with pitted 

 ones, in the Caryophyllese, and may often be found in herbaceous Dicotyledons, 

 which have been comparatively little investigated. The wood of the Mamillariae and 

 of species of Echinocactus and Melocactus contains only spiral and annular trachese, 

 and in fact both vessels and tracheides : some have feebler thickening fibres, the' 



1 Sanio, Ueberdie im Winter Starke fuhrenden Zellen des Holzkorpers., Halle (Linnsea), 1858. 

 — Id Botan. Zeitg. 1863, P- 85, &c. The latter also contains detailed citations of the older literature. 

 " Compare Chapter IV. 

 ' Compare Regnault, Ann. Sci. Nat. 4 ser. torn. XIV. p. 87. 



