44 Thompson — The Structure and Development of 



cells, which appear in longitudinal section as clear refractive 

 fibres of considerable length, lies on the outer margin of the 

 vascular bundle portion of the stem. The bundle cylinder 

 consists first of a zone of external phloem about six cells deep. 

 Most of the cells are still embryonic, with large nuclei and 

 abundant protoplasm, some few have differentiated into sieve 

 tubes. In longitudinal section the sieve plates can be recog- 

 nized. The septa are large, transversely placed, and bear 

 either four or three sieve plates with numerous perforations. 

 The cambium layer is clearly defined by its regular brick- 

 shaped cells with large nuclei. 



The wood is a broad zone, occupying more than a third of 

 the area of the section, and is traversed radially by the 

 oblong, deeply pitted cells of the medullary rays. A longitu- 

 dinal section through the wood shows numerous spiral tracheae 

 in the inner or protoxylem region ; external to this are both 

 short and long tracheids, whose walls are thickened and 

 deeply pitted. Large vessels are numerous in the outer por- 

 tion of the zone. 



On the inner side of the wood lie four large rounded patches 

 of internal phloem extending into the pith. These patches 

 are two to three times broader than the external phloem zone, 

 and consist also of sieve tubes and undifferentiated phloem 

 elements. The inner margins of the phloem patches are 

 bounded by a two-celled layer, which may be termed a phloem 

 sheath (Plate IX, Fig. i, /. s.\ This is sharply differentiated 

 alike from the adjoining pith cells and from the phloem. A row 

 of somewhat similar but smaller cells separates the outer mar- 

 gin of the phloem patches from the wood, and immediately in- 

 ternal to this row are the patches of medullary cambium. The 

 cambium cells have the usual brick-like shape, thin walls and 

 large nuclei. The cells of the sheath are rounded and in 

 close contact with each other. They have thickened pitted 

 walls and are conspicuous by their size and the large amount 

 of chlorophyll and starch they contain. The pith cells are 



