4*. ^z- 



^■■r--i!:-c 



MEDULLA. 



[ 419 ] 



MEDULLARY RAYS. 



The only circumstance which requires at- 

 ntion in the use of this table is the position 

  the decimal point. Thus, in the above 

 easure of 0*75™™, which, when broken up, 

 akes 07°^°^ and 0-05^'^\ if the first value 

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

 3uld have been, according to the table, 

 275595 Eng. inch ; but this is 10 times too 

 uch, or = 7 whole millimetres ; hence the 

 ifting of the decimal pomt, and so on. To 

 press the mode of proceeding by rule, — the 

 icimal point in the fraction of an English 

 ch given by the table should be shifted to 

 e left and as many cyphers added as there 

 e decimal places in the foreign measui'e. 

 BiBL. That in the Introduction, p. xl; 

 abertson, Edinb. Monthly Journ. 1852, 

 95; Harting, ibid. p. 453. 

 MEDULLA OF Plants.— The nameap- 

 ied by the older authors to the pith of Dico- 

 ledon (fig. 4 59 ill), from a supposed analogy 



Fig. 459. 



Horizontal section of a yearling shoot of a Dicotyledon, 

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



Magnified 25 diameters. 



thihemeduUa spinalis oi 2in\msih. ItafFords 

 ry excellent subjects for preparing sections 

 regular parenchymatous tissues, as in the 

 ler, and in the tall annual stems of 

 my of the larger perennial herba- 

 ous plants. It sometimes becomes 

 riously chambered as it grows older, 

 in the walnut and the jasmine, very 

 ;quenth^ however, it decays away 

 ;er a time, leaving the centre of the 

 ?m hollow ; this same hollow con- 

 :ion occurs early in fistular stems, 

 ch as those of the Umbelliferee, 

 )m the pith being torn up by rapid 

 pansion of the wood. The Mono- 

 tyledons do not generally possess 

 definite pith ; the cellular mass, in 

 lich the isolated Fibro-vascular 

 JNDLES are imbedded, answers to 

 diffused pith, or rather to the pith 

 d medullary rays collectively. It may 



beseenwellinsectionsof theflowering-stemof 

 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, 

 medulla; F, fibro-vascular bundles. 



Magnified 5 diameters. 



M, 



rushes and sedges, where also the cells are often 

 of most elegant radiating forms, leaving large 

 air-canals between them (PI. 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 \dtality, 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 laver of 

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

 these rays generally becomes much com- 

 pressed during growth, but theu' 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 



