103 



Fig. 2 ^ of a few scattered tracheids. The production of cribrose 

 tissue furthermore does not occur along the entire extent of 

 the inner surface; for the middle portion of each cambium 

 gives off only parenchyma, which thus gives rise to a multi- 

 seriate medullary ray {mr^ Figs. 1 and 2). The result is that 

 the sieve-tubes and accompanying cambiform-cells form four 

 distinct, more or less semicircular bundles as in Fig. 1. In 

 some instances one of the medullary rays just described may 

 be very narrow or fail altogether, and, in that case, all the 

 phloem, developed from one of the strips of cambium is united 

 into a single broad bundle. So far as the author's observation 

 extends both rays are never absent at once. 



The greater part of the crushed pith appears as a narrow 

 band of collapsed cell-walls extending along the maximum axis 

 of the ellipse nearly from end to end. p^ Fig. 1. With this 

 destroyed tissue the phloem does not come in direct contact , 

 being separated from it by a few cells of the pith {p) which 

 apparently retain their vitality. This median band of cell-walls 

 however does not represent all of the original pith, but simi- 

 lar though much thinner bands may often be found between 

 the cambium-strips with their small product of vascular tis- 

 sue , and the vessels of the primary vascular bundles , and here 

 the crushing of the cells has been so complete as to cause the 

 cellulose of the collapsed walls to unite into a thick, appa- 

 rently homogeneous membrane. It is further to be remarked 

 that this membrane comes in contact neither with the vessels 

 of the primary bundles nor the xylem of the anomalous for- 

 mations, being separated from both by a few parenchymatous 

 cells. 



Having thus examined in some detail the cross- section of a 

 rather young /o^^'^-stem, we may now turn our attention to 

 other stages , both earlier and later , in the development of the 

 plant, with a hope of deciding questions, which immediately 

 arise in connection with the anomalies described. The further 

 progress of the secondary thickening may in the first place be 

 traced in sections of older stems. In these it will be found 



