SECONDARY THICKENING. NORMAL DICOTYLEDONS. 4H9 



tioned on p. 458, and in many fleshy roots to be mentioned below, the entire mass 

 of secondary wood is to be regarded as a single cylindrical ligneous bundle. 



a. Medullary rays and medullary spots. 



Sect. 147. In the woods which have been principally investigated, every zone 

 possesses a large number of medullary rays of various rank. Since new secondary ones 

 ahvays arise as the ligneous mass grows in thickness, their number increases with 

 that of the successive layers. And further, as in the successive layers their size either 

 remains approximately uniform, or at least increases in a much lower degree than their 

 number, it may well be assumed that the proportion between the space occupied by 

 them, and that occupied by the ligneous strands, remains approximately the same in 

 all the successive layers. This assumption agrees with the observation that where the 

 increase in number of the medullary rays is very slight, an especially striking dilatation 

 of the original ones occurs ; e. g. Atragene. No detailed researches on this relation 

 exist. 



The number of the medullary rays in the surface of cross-section is apparently 

 about in inverse proportion to their size, i. e. to their breadth, and no doubt their 

 height also. Nordlinger ^ undertook a great number of enumerations, which, although 

 according to his own judgment they are not exactly reliable for woods with numerous 

 medullary rays, on account of the serious difficulties, and are also taken from casually 

 selected annual rings, yet give definite proportional numbers for the very different 

 conditions obtaining in the particular species. He gives, for example, in a breadth 

 of 5 millim., for Aristolochia Sipho 9, Clematis Vitalba 10, Cytisus Laburnum 19, 

 Robinia pseudacacia 20, Acer pseudoplatanus 33, Abies pectinata 37, Abies excelsa 

 44, Acer platanoides 47, Acer saccharinum 53, Quercus pedunculata 64, Alnus 

 glutinosa 78, ^sculus rubicunda 84, Euonymus europaeus, Punica Granatum 105, 

 Rhododendron maximum 140 (the highest figure determined). The averao-e 

 breadth of a medullary ray varies according to the species, by Nordlinger's measure- 

 ments {I.e.), from I ^«i (Quercus Cerris) to 0-015 'i^m (^Esculus, Buxus, Castanea, 

 Euonymus europaeus, Hamamelis, Juniperus communis, virginiana, Koelreuteria, 

 Ligustrum vulgare, &c.). It amounts to about 0-025 mii\ according to the same 

 author, e. g. in Abies pectinata, Pinus, Larix, Taxus baccata, Syringa vulgaris, &c. 

 to about 005 ™ni in Acer pseudoplatanus, dasycarpum, Juglans, Robinia pseud- 

 acacia, Sambucus nigra, &c., to about o- 1 mm jn Ailantus, Alnus incana, Cytisus 

 Laburnum, Gleditschia, Platanus acerifolia, &c. Whether the measurements were 

 made on sections of sufficient thinness for the determination of absolutely exact 

 numbers may remain undecided. 



The height of the medullary rays is not less variable in different species than 

 their breadth, but has received much less attention in the published investigations. 

 In woods without intermediate bundles, and in Clematis with one intermediate bundle 

 to each primary one, the height of the primary rays is equal to that of the internodes 

 and thus amounts to 100 or 2001^™. In the smallest secondary rays of the Abietineae 

 which are only 1-2 cells high, it scarcely exceeds 0-025 °^'^' 



* Querschnitte von Holzaiten, Band 2, p. 5. 



