192 



The cells in;ikiiiji' up these disks .-ire larye. ii-reunlar in ontline. very 

 thick-walled, liyiiitied. and contain starch in wiiitt r. Tlic cells hllin.ii' the 

 interspaces are small, regular, very thin-walled, nnliuiiitied and emiity. 



The formation of the disks takes i)lace at a very early Stage in the 

 growth of the twig: tliey may he seen just l)ack of the growing point in 

 Fig. 1. which is a longitudinal section through a young twig of Liri- 

 ode)i(]ro)i. 



The genus Moiniolia presents .some interesting moditications of this 

 type. The genus has been described a.s always having these partitions in 

 the pith: but, several have pointed out that this statement is incorrect. 

 In the examination of the American and some of the Asiatic species. I 

 have found only two. .1/. MrniHidud and .1/. faiidti. in which the fully 

 devehjped disks occurred. In all the other species examined, cells of the 

 sort descril)ed as maki)ig up the disks occurred scattered singly or in 

 small groups throughout the pith. Baillon, in his Natural History of 

 Plants, says of this: "In the rapidly developed shoots of some Magno- 

 lias Ave have seen these septa reduced to a single cell, nearly central, on 

 which all the surrounding cells of the ordinary parenchyma abut by one 

 end. bent, or drawn out in a quite peculiar fashion." 



In Fig. 2. which is a longitudinal section of a twig of .V. triprtalo, 

 these scattered groups of cells are shown; and. Fig. 3 shows the same 

 kind of cells in a cross-section of a twig of the same species. 



In Asiiniiia the disks seem to be made up of more regular and thicker- 

 walled cells than are fotmd in M<i;iii<ili(! and Lirifxlfiidroii. 



In the slender woody twigs of Xi/ssn. very strongly developed disks 

 Avere found, stronger in fact than in any other case examined. 

 Fiiiicfioii iif /tifJi of this tiii>c: — 



No satisfactory explanation of the function of tliis type of pith has 

 l)een offered. From superficial exannnation. the suggestion tliat its func- 

 tion was one of mechanical supiiort would seem reasonable: Imt. the fact 

 that the most strongly developed diaphragms were found in the strong 

 and slender twigs of Xyssn. Avhile the thick Md'nioJhi twigs with their 

 relatively large pith showed the Aveakest developnier:t of tiiis typ.e. seems 

 to indicate that the suggestion of mechanical supp(}rt is not a sufficient 

 explanation of their function. 



The second type of pith has often been mentioned and ligiu'ed in 

 species of -Ivijhins. I have also studied it in I'ti'voruriin. Celtic. MoJiro- 

 (Iciiflrrjii (Hdlcsifi). J'ursi/thid riridissiiiid. .hisuiiinun x/yr'c/r.s. I'diihiiniia. and 



