49© SECONDARV CHANGES. 



These relative mngnitudes may be better determined according to the number 

 of cells, or layers of cells, Mhich compose the medullary ray in breadth and height, 

 than !iccording to absolute measurements. Those under 0-025 '"'" ^^ breadth are no 

 doubt all of them only one cell broad (at most two or a few in the middle), and are 

 thus ' uniseriate ' as seen in tangential or transverse section, as, for example, in almost 

 all Coniferae, and generally in the narrowest of the above examples ; while in like 

 manner the broader ones are always pluri- or multiseriate. Similar rules obviously 

 hold good for the relative heights, and here also the number of cells varies, according 

 to the particular case, from very high figures, down to one or two. 



Certain woods possess medullary rays of /wo diffenmt sizes, with which differences 

 of the minute structure are also usually connected ; e.g. those of the Abietineae, to be 

 described below, which differ in the presence or absence of a resin-canal ; the broad 

 high multiseriate rays, and numerous low uniseriate ones between them in Quercus 

 and Fagus ; small secondary rays, which are triseriate in the middle, between the 

 much larger primary ones, in Casuarina \ &c. 



The medullary rays are sharply defined, and exactly fill the meshes between the 

 ligneous bundles, which run in a curved course around them. An exception to this 

 rule is described by Schacht^ in the case of the wood of the root in Araucaria 

 brasiliensis, in which the uniseriate rays, consisting of irregularly undulated cells, are 

 united by rows of similar cells, which run between the tracheides of the ligneous 

 bundles vertically, from one ray to another, lying above or below it. A further 

 exception is formed by the medullary spots to be described below. In the great 

 majority of cases the medullary rays consist only of parenchyma. In many woods 

 they collectively constitute the main mass of the parenchymatous tissue which is 

 everywhere distributed between the other elements : in many cases (Winterese) this is 

 represented by them alone, in others (Coniferae) at least to much the greatest extent. 

 The succulent parenchyma, which forms the greater part of the wood of the stem in 

 Carica and Vasconcella, is principally formed by the large-celled, broad, and high 

 medullary rays. 



Exceptions to this purely parenchymatous structure rarely occur. As such are 

 to be mentioned in the first instance the medullary rays of many Abietinea, all 

 investigated species of Pinus in the narrower sense, Cedrus, Larix, Tsuga canadensis, 

 Abies excelsa, and balsamea — and of Sciadopitys, which consist of two kinds of 

 elements, namely, parenchymatous cells, and tracheides of similar form to the latter, 

 distinguished by Hartig ^ as ' fibres.' 



I Among the Abietineae, the Pines (Pinus), Firs (Picea excelsa), Larix and Pseudotsuga, 



' have two kinds of medullary rays : larger ones, which contain, in their many-layered 



I central portion, a resin-canal which runs horizontally into the bast, and is not in communi- 



.' cation with other canals of the wood and bast ^, and smaller single-layered rays, usually 



only a few (1-12) cells or elements in height, and destitute of a resin-canal. The other 



trees mentioned have medullary rays of only one kind, and of the structure last mentioned ; 



' Compare Goppert, Linntea, Bd. XV. p. 747. — Low, Diss, de Casuarinearum .... Structura, 

 Berl. J 865. 



» Botan. Zeitg. 1S62, p. 412, Taf. XIIL 15. 



' Forstl. Culturpfl. p. 13, Taf. V. Compare also his Jahresber. (1837), P- 145- 



* Hartig, Naturgesch. d. forstl. CulUirpfl. p. 95, Taf. 5. — Von Mohl, Botan. Zeitg. 1859, p. 334, 



