FORM OF THE PLANT-CELL. 69 



exists, at particular points, an intercellular passage, which, through 

 the other intercellular passages and spaces, enables the subjacent 

 parenchyma to communicate freely with the external air. At the 

 inner opening of these intercellular passages, are placed (except in 

 Salvinia and Marchantia) two semi-lunar parenchyma-cells, with 

 their concave edges turned towards each other, which, according to 

 the amount of their turgescence, allow a greater or less space to 

 exist between them, or close the intercellular passage up alto- 

 gether. These two cells, with the intercellular openings, are called 

 stomates (stoma). 



b. Appendicular Organs. They are all found upon the surface 

 of plants, and are formed from cells. They are : 



1. Papilla, which are mere extensions of the external cell-wall, 

 in the form of little elevations, as upon the petals of flowers; or -as 

 vesicles, as in Mesembryanthemum crystallinum; or as apparent 

 hairs, as the so-called root-hairs. 



2. Hairs (Pili) consist of one or more thin- walled cells, varied 

 in their form and arrangement, and planted upon the epidermis. 

 They are simple (pili simplices), branched (p. ramosi), stellate (p. 

 stellati), scales (lepides), knobbed (p. capitati), glandular (jp.glandu- 

 liferi) if the upper cells secrete a peculiar fluid. 



3. Setce, stiff, thick- walled, pricking cells. 



4. Stings (Pili urentes) are stiff, thick-walled cells, terminating 

 either in a point or a little head turned on one side, and mostly 

 containing an irritating secretion. The cells at the base are often 

 thin-walled, club-shaped, and swollen, and the whole enclosed by a 

 number of wart -like cells produced from the epidermis. 



5. Thorns (Aculei) consist of numerous rigid, thick-walled cells, 

 firmly bound together, and terminating in a sharp point. 



6. Warts ( Verrucce), formed out of many compact semicircular 

 and variously formed cells. 



c. Cork (Suber). In the cells of the epidermis there is often 

 collected a grumous substance, from which are developed flat 

 tabular cells. The epidermis bursts, and thus is formed what is 

 popularly called " bark," or, where it is strongly developed and 

 elastic, " cork," as in juicy fruits, but especially in the second year's 

 stems of the Quercus Suber. 



d. Root Sheath ( Velamen radicum). In most tropical Orchi- 

 y and some Aroidecs, there exists upon the epidermis of the 



roots (the adventitious roots) a layer which is ordinarily composed 

 of the most delicate cellular tissue, whose contents are entirely air. 



The controversy about the nature of the epidermis was only possible 

 at a time when the conception of the elementary structure of plants was 

 of a very imperfect kind, and a false analogy with the epidermis of ani- 

 mals led to erroneous conclusions. 



"When a part is about to be formed from the cambium in phaneroga- 

 mic plants, the first thing that meets our view is a layer of one or more 

 series of delicate cells, which are homogeneous and contain a clear fluid, 



F 3 



