TISSUE SYSTEMS 



338 



Fie. 711. 



defined as flattened, membranous, more or less rounded plates, 

 attached by the centre to the epidermis, and presenting a more 

 or less irregular margin from the unequal prolongation of the 

 component cells. These scales are particularly abundant on the 

 surface of some plants, to which they communicate a scurfy 

 or silvery appearance, as in ElcBagnus. 



Other modifications of hairs which are allied to the above are 

 the ramenta or ram ciitaceous hairs so frequently found upon 

 the stems and petioles of Ferns. These consist of a layer of 

 cells combined so as to form a brownish flattened scale attached 

 by its base to the surface of the epidermis 

 from which it grows. 



Hairs must be distinguished from prickles, 

 warts, &c., which arise from the sub-epider- 

 mal tissue as well as the epidermis, and 

 which have been termed emergences. These, 

 again, should be carefully distinguished from 

 thorns, which we have seen to be abortive 

 branches. 



The ordinary hairs above described may 

 be empty, or they may contain fluid of a 

 watery nature, either colourless or coloured. 



Hairs occur upon various parts of plants ; 

 their more common position is upon the leaves, 

 stems, and young branches, but they may also 

 be found on the flower-stalks, bracts, parts of 

 the flower, the fruit, and the seed. A re- 

 markable form occurs on young roots, where 

 cells of the epidermis produce long tubular 

 outgrowths which become very closely applied to the particles 

 of soil. These are termed root-hairs [fig. 711). The hairs 

 which occur on the parts of the flower frequently play an in- 

 direct part in the process of fertilisation by collecting the pollen 

 which falls from the anthers ; hence such are termed collecting 

 hairs. The collecting hairs which occur on the style of the 

 species of Campanula are peculiar from their upper end retract- 

 ing vrithin their lower at the period of pollination. 



Besides the forms of glandular tissue already described, 

 certain secretions are formed in peculiar hairs. These may 

 arise from a single cell of the epidermis or from several. In 

 some cases the secretion is formed inside one or more cells ; in 

 others it is due to a degeneration of the cell- wall, and then accu- 

 mulates under the cuticle of the terminal cell or cells {figs. 712 



Fig. 



711. Eoot-hairs 

 on the surface of a 

 young root. 



