Anatomy and Histology. 107 



THE DEFINITIVE STEM. 

 PRIMARY STRUCTURE. 



After the tenth internode, approximately, has been laid down, the 

 stem takes on its definitive structure. The number and appearance of the 

 various structures within the growing tip vary a good deal, according to 

 the rate of growth. This is largely due to the crowding together of the 

 nodal characters, but in part also to the size of the terminal bud, and 

 therefore to the number of leaves. In a thick apex more bundles of 

 primary elements appear at a given level (plate 31, fig. 3) ; also the size 

 and frequency of branching of the canals is greater within a given zone 

 (plate 36, fig. 5). For the purpose of description it will serve to present 

 briefly the differences observed, at various levels of a stem one year old, of 

 normal growth-rate in the field, as this will give an epitome of the develop- 

 ment of the tissues. The specimen before me is a twig which grew in 1908, 

 collected at the close of its elongation for that season. It is 11 cm. long, 

 4 mm. in diameter at the base, and 1.6 mm. just behind the tip. The 

 structure at the levels mentioned is as follows: 



Within the last millimeter of the tip one finds the vascular tissues 

 undifferentiated, though the medulla and vascular zone are recognizable. 

 The primary cortical canals appear opposite median leaf-traces, 1 but 

 nowhere else (plate 38, fig. 1 ; plate 31, fig. 5). The starch sheath (endo- 

 dermis) is recognizable only by the starch-content, which appears only 

 opposite leaf-traces, while starch is absent from the endodermis else- 

 where. Within half a millimeter further down, at a diameter of a milli- 

 meter, all the 5 medullary canals appear, 17 vascular bundles are distinct, 

 and 1 6 cortical canals are present (plate 3 1 , fig. 4) . In perhaps half of the 

 bundles spiral vessels have developed. These are in curved plates of 1 to 3 

 vessels, each separated by wood-parenchyma. The epidermis is densely 

 clothed with T-shaped trichomes. The endodermis may be traced com- 

 pletely around the stele, on account of its starch. At this level may be seen 

 the earliest indications of the stereome bundles in the primary leptome and 

 in the pith. 



At 10 mm. from the apex (diameter 2.5 mm.) the collenchyma of 4 to 

 6 rows is well developed. The characteristic thickening is first seen in the 

 periclinal walls, and these become still more conspicuously thickened 

 in the later stages. The larger bundles have xylem plates 6 to 8 cells 

 deep radially. Interfascicular cambium is being developed. The stere- 

 ome is still thin-walled, but the definitive size of the cells has been reached, 

 and thickening has taken place at the angles. In the section before me I 

 count 2 5 primary cortical canals and 1 o medullary canals. The section was 

 evidently taken just above the plane in which the pith-canals branched, as 

 two of the canals are cut at the fork. 



At 15 mm. (diameter 3 mm.) mechanical elements have appeared in 

 the hadrome, and the stereome is more advanced as to the thickening of 

 the walls. The collenchyma has been somewhat stretched periclinally, 

 the walls so placed being much thicker. The walls of both cortical and 



1 Very occasionally a pair, a single one on each side of the trace, occurs. 



