HISTOLOGY OF STEM OF THE WAX PLANT 271 



an unbroken circle, while in some of the older portions the parenchyma 

 has reached almost to the pith and the distance separating them is 

 much thinner than the band of wood in the young stem. 



In a tangential section the masses of parenchyma are shown to 

 be lenticular in shape with their longest axis parallel to the axis of the 

 stem, looking very much like medullary rays, which I at first took them 

 to be, as none of the other cells in such a section show the ordinary 

 appearance so generally characteristic of these tissues. If, however, 

 we make a radial section, we can distinguish four well-marked forms 

 of cells: (i) the wood cells proper; (2) the ducts; (3) the cells 

 of the parenchyma, if the section has been made through a portion of 

 the stem containing them; (4) rectangular cells, very regular in 

 form and length, their ends thus forming well-marked lines across 

 the section. These, I think, are the medullary rays. 



Next to the pith will be seen cells with thickened walls giving them 

 the appearance of a spiral. These latter are best seen in a section of 

 a young stem. These various tissues with the exception of the paren- 

 chyma all exhibit the characteristic reactions of lignified cells, staining 

 with the various reagents used for the purpose of identifying them, such 

 as phlorglucin and thymol. As far as my investigation has gone, the 

 ducts contain only air. The medullary rays and parenchyma are 

 usually filled with starch ; the wood cells sometimes contain the latter, 

 but are generally empty. The position of the starch varies with the 

 season of the year and the circumstances under which the plant has 

 been kept. 



Ducts 



In a transverse section of the stem these will be noticed scattered 

 through the wood. Their diameter is much greater than that of the 

 wood cells, being from 25 to 50 ^l, while the latter are quite constant 

 in size, having a cross section of about 15 At. The length of the ducts 

 varies greatly, some being 100 /-t long and others only 10 yit. They 

 form large canals through the stems, the ends meeting sometimes 

 at right angles, but generally overlap, making an angle of about 40° 

 with the direction of their length. Their walls are about equal in 

 thickness to the walls of the wood cells, but are regularly pitted as 

 shown on pi. II, fig. 3. 



Sachs, in his ''Physiology of Plants," states that Theodore Hartig, 



