6 MINNESOTA BOTANICAL STUDIES. 



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

 consisting of more or less interlacing hyphae, 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- 20). 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. jo) when seen in cross section. A much greater devel- 

 opment of gelatinous material occurs in the pith web {Fig. Ji). 

 In this region most of the hyphae 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 an}' plane, will show the hyphae 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 

 II.) ; numerous short plications extend longitudinally and also 

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

 {Fig. J2), 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. jj). 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 3'oung air vesicles {Fig. J4). The 

 cells of the epidermis and hypoderma are frequently well filled 

 with granular reserve carbohydrates. No important differences 



