STELAR THEORIES 



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zeylanica (Farmer and Freeman, 19) this tissue does not appear 

 in the stem of the young plant, while its appearance in the 

 vascular strands of older plants is fitful and irregular. 2 In 

 Botrychium lunaria (Van Tieghem, 67) an external endodermis 

 only is present in the young stems ; hence it may be termed 

 monostelic. In slightly older stems an internal endodermis 

 appears, and thus the structure becomes astelic, and ultimately 

 gamodesmic. In Botrychium virginianum (Jeffrey, 34) no internal 

 endodermis is present, so that the stem is monostelic. Hence, 

 in closely related plants, a great difference in the structure — 

 considered from the morphological point of view — results from 

 the acceptance of the endodermis as a layer of morphological 



value. This same irregularity is exhibited in the Osmundaceae 

 (Faull, 21). This difficulty was fully recognised by Strasburger, 

 who suggested that the term "endodermis" should be used 

 in the physiological sense, and restricted to that layer of cells 

 which exhibits the well-known cuticularisation ; while the term 

 " phlceoterma " should be used to designate the band of tissue 

 delimiting the cortex from the stele. Here again arises a 

 difficulty, for the only means of distinguishing the phlceoterma 

 is that of position. 



This difficulty associated with the endodermis may be 

 illustrated by fig. 5, a, which represents a longitudinal section 

 of a fern stem which has an internal endodermis originating 

 in connection with the first leaf-gap. Taking the region below 



2 See also Lang, Annals of Botany, xvi. 



