MEDULLA. 



[ 495 ] MEDULLARY RAYS. 



fraction to be converted into parts of an 

 Eno-litih inch luiL-t be broken up into its 

 decimal parts, and each valued separately 

 from the Table; thus, to convert 0-75""" into 

 a fraction of an Eu'^lish inch — 



(by tbe Table). 



0.7mm _ 0-0275-j9o 



O-Oo""" = 0-00 1968.53 



0.75mm _ 0-02952803 Eng. inch. 



The only circumstance which requires 

 attention iii the use of this Table is the 

 position of the decimal point. Thus, in the 

 above measure of O-ro'""^ which, when 

 broken up, makes O"?'"'" and O-Oo"""*, if the 

 first value (0-7) had been 7-0, the value in 

 Eng. inch would have been, according to 

 the Table, 275595 Eng. inch ; but this is 

 10 times too much, or = 7 whole milli- 

 metres ; 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 manj- ciphers added as there are decimal 

 places in the foreign measure. 



Harting introduced a new measure, the 

 .j^t__th of a millimetre = •0000a9 in., called 

 a micromillimetre, expressed by /x. This 

 and its multiples are often used in foreign 

 works. The /x is nearly equal to the tsW 

 of an English inch. But we agTee with 

 Frey, that its use possesses no advantage. 



Throughout this work the foot and inch 

 and their fractional parts are expressed for 

 brevity by placing respectively one or two 

 acute accents on their right side : thus, one 

 foot is denoted bv 1', one inch by 1", and 

 ' ' ^ ' 10". 



.j^^th of an inch bv 1 



MEDUL'LA OF Plants. — The 



name 



formerly applied to tlie pith of Dicotyledons 

 (fig. 455 Mj, from a supposed analogy with 



Fio'. 455. 



the medulla epinalis of animals. It affords 

 excellent sections of regular parenchyma- 

 tous tissues, as in the elder and in the tall 

 annual stems of many of the larger peren- 

 nial herbaceous plants. It sometimes be- 

 comes curiously chambered as it grows 

 older, as in the walnut and the jasmine ; 

 very frequently, however, it decays after a 

 time, leaving the centi-e of the stem hollow; 

 this same hollow condition occurs early in 

 tistular stems, such as tbose of the Um- 

 belUferse, from the pith being torn up by 

 rapid expansion of the wood. The Mono- 

 cotyledons do not generally possess a defi- 

 nite pitli ; the cellular mass, in which the 

 isolated Fibeo-vascular Bundles are 

 imbedded, answers to a diffused pith, or 

 rather to the pith and medullary rays col- 

 lectively. It may be seen well in sections 

 of the flowering stem of lilies (fig. 456 M). 



Fig. 456. 



Horizontal section of a yearling shoot of a Dicotyle- 

 don. M.iQedulla; RM, medullary rays ; T, medullary 

 sheath. Magnified 25 diameters. 



Horizontal section of the Howering stem of a lily. 

 M, medulla ; F, fibro- vascular bundles. 



Magnified 5 diameters. 



A more definite medulla occurs in the stem 

 and leaves of the rushes and sedges, where 

 also the cells are often elegant radiating 

 forms, leaving large air-canals between 

 them (PI. 47. fig. 18). The pith of a Dico- 

 tyledonous stem loses itself gi-adually in 

 the terminal bud, where it is confounded 

 with the nascent wood and cortical layers. 

 In this stage its cells possess an active 

 \-italitv, which, however, is soon lost. 



ME'DUL'LARY RAYS.— The processes 

 of cellular tissue extending outwards from 

 the pith, between the fibro-vascular bundles 

 of a Dicotyledonous stem in the fir.^t year 

 of gi-owth (fig. 455 RM), together with 

 additional interpose'd rays formed between 

 the older in each succeeding annual layer 

 of wood (fig. 457, 1, 2, 3, 4). The tissue of 

 these rays generally becomes much com- 

 pressed durino: gi'owth ; but their size and 

 the degree of development differ much in 



