Internal PlUocni in Gclseniium sempcnnrens. Ait. 45 



much larger, have thin but slightly pitted walls, and a scanty- 

 supply of chlorophyll and starch, while the intercellular spaces 

 are larger than those of the phloem sheath. A few short 

 sclerenchymatous or " stone " cells are sometimes present. 

 Very early in the life history of the stem death of the pith 

 cells occurs. The cell contents dry up, the pith as a whole 

 shrinks away from the sides and becomes detached from the 

 phloem sheath, but persists as an inert somewhat lignified 

 mass, until its place is usurped by the enlarging phloem 

 patches. 



Histology of the Stem from the Second to the Tenth 



Year. 



In a transverse section of a stem at the end of the second 

 year's growth, the most prominent change is the increased 

 size of the internal phloem patches. Each has pushed farther 

 out into the pith, and as the growth has been greater in the 

 middle than at the sides, the inner margin has a curved out- 

 line, with the convexity toward the pith. The formation of 

 new cells from the medullary cambium takes place centrifu- 

 gally, the newly formed cells lying external to the old. On 

 the inner side of each patch, adjoining the phloem sheath, a 

 dark crescentic mass of partially obliterated tissue is now evi- 

 dent. This is composed of the older sieve tubes that have 

 collapsed and been pushed together by the pressure from the 

 new elements laid down by the active medullary cambium. 



The external phloem has increased but little in breadth, in 

 comparison with the internal patches, but the total number of 

 cells and the actual area of the zone is greater than before. 

 Here and there along the border are darker areas, composed 

 of four or five compressed cells, showing that the same crowd- 

 ing and obliteration goes on, although to a less extent than in 

 the internal patches. 



In older stems, the increased size of the internal phloem 

 patches becomes more and more prominent. The masses of 



