6 MINNESOTA BOTANICAL STUDIES. 



ened walls. The central part of the main stipe is a pith web 

 consisting of more or less interlacing hyphse, showing muci- 

 laginous thickening of the walls. It is much the same as the 

 pith web of the rachis and lamina. 



Rachis. This is a strap-like structure, rough-tuberculate on 

 both surfaces. The elevations are to be considered as emer- 

 gences consisting of cortical and hypodermal cells covered with 

 epidermis. About one third of the entire thickness of the rachis 

 is embraced in the pith web (Fig- 28). The epidermis consists- 

 of thin-walled prismatic cells with slightly thickened outer wall. 

 Chlorophyll bodies are present here and in the hypoderma ; the 

 cells of the latter tissue resemble those of the epidermis in ap- 

 pearance (Fig. 29). There is a gradual transition to the cortex 

 where the cells are thicker walled and elongated in the direc- 

 tion of the long axis of the rachis. Here chlorophyll bodies 

 are absent. A somewhat well-marked limit is seen between 

 cortex and pith web. The gelatinous thickening of the cell 

 walls of the inner cortex gives an appearance of collenchyma 

 (Fig. 30) when seen in cross section. A much greater devel- 

 opment of gelatinous material occurs in the pith web (Fig* Jf)- 

 In this region most of the hyphge extend longitudinally, but 

 there are many also passing horizontally and about as many in 

 the direction of the thickness of the strap. Thus a section of 

 the pith web, cut in any plane, will show the hyphse extending 

 in various directions ; some may be followed for a distance, 

 others are cut straight across and some obliquely. 



Lamina. This is very much roughened externally (Plate 

 //.) ; numerous short plications extend longitudinally and also 

 in part obliquely. The pith web is elevated at these places 

 (Fig, 32), so that the emergences are deep-seated and not 

 merely cortical as in the rachis. There are no other structural 

 differences between lamina and rachis ; epidermis, hypoderma, 

 cortex and pith are essentially similar in the two regions. 

 There is no real pith web in the proliferations, but the cells of 

 the medullary region often show a certain amount of thicken- 

 ing (Fig. 33). The cells of the epidermis are generally short, 

 but in some cases rather tall prismatic, just as in the main 

 rachis and lamina. The tall prismatic cells are found regu- 

 larly in the epidermis of young air vesicles (Fig. 34). The 

 cells of the epidermis and hypoderma are frequently well filled 

 with granular reserve carbohydrates. No important differences- 



