MEDULLA. 



[ 419 ] 



MEDULLARY RAYS. 



The only circumstance which requires at- 

 tention in the use of this table is the position 

 of the decimal point. Thus, in the above 

 measure of 0'75 mm , which, when broken up, 

 makes 07 mm and 0'05 mm , if the first value 

 (0'7) had been 7'0, the value in Eng. inch 

 would have been, according to the table, 

 0-275595 Eng. inch ; but this is 10 times too 

 much, or = 7 whole millimetres ; hence the 

 shifting of the decimal point, and so on. To 

 express the mode of proceeding by rule, the 

 decimal point in the fraction of an English 

 inch given by the table should be shifted to 

 the left and as many cyphers added as there 

 are decimal places in the foreign measure. 



BIBL. That in the INTRODUCTION, p.xl; 

 Robertson, Edinb. Monthly Journ. 1852, 

 p. 95 ; Harting, ibid. p. 453. 



MEDULLA OF PLANTS. The name ap- 

 plied by the older authors to the pith of Dico- 

 tyledon (fig. 4 59 M), from a supposed analogy 



Fig. 459. 



Horizontal section of a yearling shoot of a Dicotyledon. 

 M, medulla ; RM , medullary rays ; T, medullary sheath. 



Magnified 25 diameters. 



with the medulla spinalis of animals . It affords 

 very excellent subjects for preparing sections 

 of regular parenchymatous tissues, as in the 

 elder, and in the tall annual stems of 

 many of the larger perennial herba- 

 ceous plants. It sometimes becomes 

 curiously chambered as it grows older, 

 as in the walnut and the jasmine, very 

 frequently, however, it decays away 

 after a time, leaving the centre of the 

 stem hollow ; this same hollow con- 

 dition occurs early in fistular stems, 

 such as those of the Umbelliferae, 

 from the pith being torn up by rapid 

 expansion of the wood. The Mono- 

 cotyledons do not generally possess 

 a definite pith ; the cellular mass, in 

 which the isolated FIBRO-VASCULAR 

 BUNDLES are imbedded, answers to 

 a diffused pith, or rather to the pith 

 and medullaryrayscollectively. It may 



be seen well in sections of the flowering-stem of 

 lilies (fig. 460 M}. A more definite medulla 

 occurs in the stem (and in the leaves) of the 



Fig. 460. 



Horizontal section of a flowering- stem of a lily. M, 

 medulla ; F, fibro-vascular bundles. 



Magnified 5 diameters. 



rushes and sedges, where also the cells are often 

 of most elegant radiating forms, leaving large 

 air-canals between them (PL 38. fig. 18). 

 The pith of a Dicotyledonous stem loses 

 itself gradually in the terminal bud, where it 

 is confounded with the nascent wood and 

 cortical layers. In this stage its cells pos- 

 sess an active vitality, which, however, is 

 soon lost. 



BIBL. General Works on Structural 

 Botany. 



MEDULLARY RAYS. The processes 

 of cellular tissue extending out from the 

 pith between the fibro-vascular bundles of a 

 Dicotyledonous stem in the first year of 

 growth (fig. 459 R M), together with addi- 

 tional interposed rays formed between 

 the older in each succeeding annual layer of 

 wood (fig. 461 1, 2, 3, 4). The tissue of 

 these rays generally becomes much com- 

 pressed during growth, but their size and 



Fig. 461. 



Section of a four- years' old shoot of the Cork oak. M, pith ; 

 1 , 2, 3, 4, medullary rays of successive years ; P. C, liber layers ; 

 S, cork layers. 



Magnified 20 diameters. 



2E2 



