4.6 Thompson — The Structure and Development of 



crushed tissue, or " Hornbast," (Plate IX, Fig. 2) are more 

 numerous and broader, the later formed ones lying in concen- 

 tric layers external to the older masses. Some large phloem 

 parenchyma cells are often present between the crushed masses, 

 for they are better able to resist the crushing process, owing to 

 their greater turgidity. The patches may thus present a strati- 

 fied appearance from the alternation of the bands of crushed 

 tissue and the scattered parenchyma cells. Each of the four 

 patches usually divides into two parts, so that in the oldest 

 stems eight cone-shaped masses of internal phloem are present. 

 The neighboring patches grow together laterally, while they 

 continue to encroach upon the pith. In the oldest stem ex- 

 amined (Fig. 2), of about twelve years' growth, the internal 

 phloem patches entirely fill the former pith area, except a 

 very small space in the centre, where a shrunken thread of 

 dead tissue represents all that remains of the pith. The 

 patches by this time are composed almost wholly of " Horn- 

 bast." Only a few sieve tubes are distinguishable, and these 

 are more or less distorted. The contrast between the large 

 cells of the phloem sheath and the dark crushed masses is 

 very striking. 



The breadth of the external phloem, which during the first 

 few years was less than that of the internal patches, increases 

 greatly in older stems. In a six year old stem its breadth 

 almost equals that of the patches, in a ten year old stem it 

 exceeds them. The same alternation of bands of " Horn- 

 bast " with parenchyma cells occurs as in the internal patches, 

 but as the pressure conditions are different here the bands are 

 narrower and less marked. As the growth has been centripe- 

 tal, the newly formed tissue lies internal to the old. 



The widening of the medullary rays is very noticeable in 

 older stems. The width of a ray at the periphery of the wood 

 is six or eight times greater than at the centre. Elongated 

 cells, that are continuations of the rays, separate the cone-like 

 masses of the external phloem zone. 



