THE EPIDERMIS 



127 



guish between chloroplastids and leucoplastids in the altered state 

 which necessarily results from the condition of fossilization. 



In some instances the epidermis commonly becomes a multiple 

 layer often known as water tissue. This is particularly the case 

 with plants of different systematic affinities which are exposed to 

 extremely arid conditions. Ficus elastica affords an excellent 

 illustration of the multiplication of the epidermal layer in leaves 



FIG. 95. Epidermis of the Urticaceae. Explanation in the text 



of xerophytic habit. Here the epidermis, which in the young con- 

 dition consists of a single stratum, as the leaf matures becomes 

 divided into several stories of cells, all characterized, like the epider- 

 mis from which they are derived, by the absence of chloroplastids. 

 Another feature which especially distinguishes the epidermal 

 structures of the Urticaceae and Acanthaceae is the presence 

 of deposits of carbonate of lime, generally in the form of acinose 

 masses supported on a peduncle from one side of the cell. In 

 urticaceous leaves which are mesophytic in their environment, 

 such as the nettle or hemp, the structures under discussion, known 

 as cystoliths, are found in the outermost cells of the plant that is, 



