136 IX. THE DICOTYLEDONOUS STEM HERBACEOUS TYPES. 



containing inner cortex. Then comes the collenchyma ring, and 

 we can determine that the elements composing it are many times 

 as long as broad, and join on to one another with oblique end 

 walls. 



If we have an opportunity of examining cross-sections of an 

 older stem of Aristolochia it may not be amiss to get in it a 

 general idea of the results of cambial action and the other 

 changes which have taken place. We shall see that the pith is 

 partially destroyed, and instead of preserving its cylindrical 

 form, as it generally does, has in this case become flattened more 

 or less, perhaps into a mere streak. Through the activity of 

 the cambium the primary vascular bundles have increased con- 

 siderably in size, and the interfascicular cambium has formed 

 between them secondary bundles, which make up the woody 

 ring. The medullary rays are narrow, and more numerous, the 

 cambium adding to their length proportionally to the growth 

 of the bundles. The cells of the medullary rays are radially 

 elongated. If the flattening of the pith has proceeded to a great 

 extent, nearly all of the medullary rays will appear more or less 

 curved ; at one end, leaving the pith at right angles, while the 

 other, if prolonged, would end at right angles to the epidermis. 

 The cambium layer is very recognisable, and the bast will be 

 seen to have also increased in quantity. The sclerenchyma 

 layer may no longer be continuous, but broken up, by the 

 increase of the inner tissues, into isolated greenish-looking 

 strands. We w,ill not, however, further concern ourselves with 

 the structural features of Aristolochia, as we propose to study 

 the subsequent thickening of stems hereafter in other material. 



If the stem of Aristolochia Sipho is not available, almost 

 equally good material can be found in the young seedling stems 

 of Ricinus communis, which can be grown for the purpose ; as 

 also can the Sunflower (Helianthus annum), or the Jerusalem 

 Artichoke (H. tuber osus), the latter being grown from seed or 

 tubers. The deviations from the structure detailed above will 

 offer no difficulties. 



