THE EPIDERMAL SYSTEM. 99 



which, as pointed out by Professor Beal,* are drawn out the long 

 thread-like lashes which are so abundant on the leaves of some thistles 

 and other Composites (Fig. 88). These lashes appear to be of the na- 

 ture of secretions, and they are capable of being drawn out to an aston- 

 ishing length. These are, in turn, much like the glandular hairs on 

 the leaves of Dipsacm sylvestris, discovered by Francis Darwin.f 

 and from which motile protoplasmic filaments protrude. Mr. Darwin 

 concludes that they have the power of absorbing nitrogenous matter. 



130. Stomata (singular, /Stoma). These structures con- 

 sist, in most cases, of two specially modified chlorophyll- 

 bearing cells, called the Guard-cells, which have between 

 them a cleft or slit passing through the epidermis (Figs. 89, 

 90). These openings are always placed directly over interior 

 intercellular spaces. Stomata are . developed from, and in 

 their distribution always have a relation to, the epidermal 

 cells ; in an epidermis composed of regular cells there is 

 more or less regularity in the arrangement of the stonlata ; 

 but when tne epidermal cells are irregular the stomata are 

 also irregularly placed. 



They occur on aerial leaves and stems most abundantly, 

 being sometimes exceedingly numerous, and are exception- 

 ally found on other parts, as the sepals, petals, and carpels 

 of the flowers. On submerged or underground stems and 

 leaves they are found in less numbers, and from true roots 

 they are always absent. The stomata on leaves are generally 

 confined to the lower surface, and when present on the up- 

 per they are usually much fewer in number ; there are, how- 

 ever, some exceptions to this. 



131. Their development generally takes place in the fol- 

 lowing way : in a young epidermis-cell a partition forms at 

 right angles to the plane of the epidermis, cutting off a por- 

 tion of the cell ; this in one series of cases becomes the 

 mother-cell of the stoma ; in another series of cases, how- 

 ever, it is divided one or more times by subsequent partitions 

 before the mother-cell is formed. In either case, when once 



* In an article entitled "How Thistles Spin," in the American Nat- 

 uralist, 1878, page 643. See also an article by the same writer on 

 " Hairs and Glandular Hairs of Plants : their Forms and 'Uses," in the 

 same volume of the journal named, on page 271. 



f See his account, with a plate, in Qr. Jour, of Mic. Science, 1877, p. 245. 



