70 



ON THE PLANT-CELL. 



and which form the external boundary and cover over the developing part. 

 These cells of various signification I call Epithelium (fig. 69. a). The 



69 



70 



same thing may be observed in the so-called vascular cryptogamic plants 

 (Ferns, Lycopodiacece, Equisetacece, Rhizocarpeai). It is also seen in 

 the MarchantiacecB. This epithelium differs according to the external 



influences which act upon it. It is 

 71 only in a few cases that the epithelium 



retains its true character for any length 

 of time, as in the cavities of the ovary. 

 In the air, in water, or in the earth, it 

 becomes changed more or less, the cells 

 become more compact, and their ex- 

 ternal surface flattened (fig. 70. a) in the 

 air, so that most epidermis-cells have 

 a tabular or ligulate form (fig. 71. a). 

 The forms which these tissues assume are very numerous. In the 

 more delicate forms of these external coverings, as seen in the petals of 

 some plants, individual cells elevate themselves above the surface (fig. 

 72. a), and thus form a transition to the simple forms of hairs. 



7:* 



72 



In other petals they are much more compact, and very much elongated 

 from within outwards (fig. 73. a). The extreme of these two conditions 

 occurs in the epidermis of some seeds, especially in those of the Legumi- 

 nosce. Here the cells are often long, cylindrical, extended from within 

 outwards, and often entirely filled up at particular points (fig. 74. a). 



69 Epithelium (a) from the gemmules of Tradescantia crassula, with a layer of 

 parenchyma-cells lying under it. 



70 Epiblema (a) from the root of Spirodela polyrrhiza, lying over a layer of paren- 

 chyma-cells. 



71 Epidermis (a) from the upper surface of the leaf of Tradescantia discolor, with the 

 parenchyma beneath. 



" Papillary epidermis (a) from the under surface of the petals of Iris variegata, 

 accompanying the underlying parenchyma. 



73 Epidermis (a) of the under surface of the petals of the white rose. The external 

 surface is beset with delicate furrows (aciculatus). Loose parenchyma underneath the 

 epidermis. 



