STRONG DRINK AND TOBACCO SMOKE. 13 



cells that have coalesced to form long delicate fibres 

 (Plate 1, fig. 19). Stringy, tough vegetables, with which 

 we sometimes make unpleasant acquaintance, owe their 

 unpalatableness to the large quantity of fibrous tissue 

 which they contain. 



The outer coverings or skins of herbaceous plants are 

 very frequently composed of thick-sided, flat, tabular 

 cells (figs. 20, 21). The upper surfaces of leaves are 

 frequently covered with a thin pellicle of cellular tissue 

 (fig. 22) ; their lower surfaces with a tissue composed 

 of flat, tabular cells with wavy outlines (fig. 23). 



Many readers will be surprised to learn that plants 

 breathe principally through their leaves, but the experi- 

 ments of philosophers have placed the fact beyond 

 question. The delicate organisms by which this function 

 is performed may be well seen by detaching a minute 

 piece of skin from the under side of a leaf, and placing it 

 in water under the microscope. These will be seen lying 

 in great profusion amongst the peculiar flat tabular cells, 

 with undulating outlines (fig. 23), peculiar to this part, 

 as minute, kidney-shaped cells, touching at their ends 

 and having a slit or minute opening between them. 

 One of these, viewed from above, and greatly mag- 

 nified, is seen at fig. 24, and cut through vertically at 

 one point of their junction at fig. 25. As moisture 

 is supplied to or withdrawn from the leaves either 

 by imbibition or evaporation, these breathing pores 

 (stomates as they are called) expand or contract, and 



