SECONDARY THICKENING. NORMAL DICOTFLEDONS. 517 



The far more frequent case mentioned under (i) b., to which we shall have to return 

 in considering the changes of bast and cortex, occurs, for example, in the root of 

 Taraxacum, Rubia, and Umbelliferae. A root of Taraxacum which is now before me, 

 ^mm in thickness, has, for example, a ligneous body only about 0-5"^™ in diameter. 



Lastly, case (2), which especially belongs to the present subject, occurs in its 

 most typical form in Brassica and Raphanus. The main mass of the Radish and 

 the Turnip is formed of the chiefly parenchymatous wood ; the bast and external ^ 

 cortex are not more than 1-2™^! thick. Between the typical cases mentioned under 

 {i)i>. and (2), a number of intermediate forms occur, with but slight difference 

 between the mass of the wood and that of the bast and external cortex, e.g. roots of 

 the Umbelliferse, Scorzonera hispanica, Rheum Rhaponticum, &c. But in these 

 cases, as the relative thickness of the wood increases, the proportion of parenchyma 

 to the specific woody elements increases in favour of the former, if it is allowable to 

 enunciate a general rule for cases which show such great variety in detail. 



In the roots in question, the tracheae of the wood, in all known cases, are 

 exclusively vessels, w^ith a wall which is reticulately thickened (and then often with 

 scalariform transverse meshes), or has bordered pits, the latter structure being not 

 uncommonly found on the surfaces which abut on other vessels, the former on those 

 adjoining non-equivalent tissue. Their average width is considerable ; w'ider and 

 much narrower ones frequently occur together. They are always immediately 

 accompanied by rows of elongated prismatic cells with pointed or horizontal ends, 

 which may be called fibrous cells In the sense defined above, and the nature of the 

 contents of which still requires more accurate comparative investigation ; they are 

 further accompanied by short-celled parenchyma, which in form and position corre- 

 sponds to the bundle-parenchyma. Tracheides do not seem to occur, yet this point 

 also still needs further investigation. Between the ligneous bundles, which are thus 

 constructed according to the general rule, medullary rays of different orders are 

 inserted as the wood becomes more strongly developed, in a manner which likewise 

 corresponds to the general rule. So far as is known these are always parenchy- 

 matous, and their elements are generally to be distinguished from those of the 

 bundle-parenchyma by their form and position, and the special nature of their 

 contents, according to the rules described above; but in the particular cases the 

 dififerences may be either very clear or not distinct. Leaving out of consideration 

 the cases marked {i)a. where the ligneous body is quite small, the formation of massive 

 parenchyma is apportioned between the ligneous bundle and the medullary rays in 

 two principal forms, between which of course there are again many intermediate 

 cases. 



(i) Narrow ligneous bundles are separated or divided up by broad parenchy- 

 matous medullary rays. They consist principally of vessels and fibrous cells, the 

 latter being usually narrow, thick-walled, and lignified ; the medullary rays are thick 

 masses of parenchyma, their cell-walls for the most part thin and not lignified. The 

 above-mentioned cases, with strongly developed main medullary rays, belong to this 

 category, as Urtica, Cucurbita, Symphytum officinale, &c. Comp. p. 474, Figs. 

 203, 204. 



(2) In most really fleshy roots the main mass of the parenchyma in the ligneous 

 body belongs to the ligneous bundle itself. In its most internal portion, bordering 



