64 PART [. THE MORPHOLOGY OF PLANTS. [ 41. 



Roots are occasionally developed as thorns, as in the Palms 

 Acanthorhiza and Iriartea, and in Myrmecodia (Rubiacece). 



In some species of Jussioea (e.g. J. repens) which live in swamps, 

 some of the adventitious roots develope into floats, containing large 

 intercellular spaces. 



14. Hairs and Emergences. Under these terms are 

 included various appendages of a lower morphological value than 

 the stem, the leaf, or the root, upon all of which they may be 

 borne. 



They are frequently of adventitious origin, and are commonly 

 not developed in any definite order of succession. But when they 

 are developed in progressive succession, as in the gametophyte of 

 some of the lower plants, it is by no means easy to certainly 

 distinguish them from other members of the body. For instance, 

 in certain Liverworts (e.g. Riccia, Marchantia) the lower (ventral) 

 surface of the thalloid gametophyte bears one or two rows of 

 scales : from their regular arrangement these ventral scales might 

 be regarded as leaves, were it not that in other forms (e.g. Corsinia, 

 Clevea) they are arranged quite irregularly. The close relation 

 between hairs and leaves in the gametophyte is further indicated 

 by the fact that in some foliose Liverworts (e.g. Jungermannia 

 bicuspidata) the ventral leaves (amphigastria) of other forms are 

 each represented by a hair. 



(a) Hairs. Hairs are always developed from superficial cells ; a 

 hair usually takes origin from a single superficial cell, but some- 

 times from more than one. Their growth is generally apical, 

 but sometimes basal. 



Hairs vary in form and structure; they may be unicellular, 

 when they are termed simple ; or multicellular, when they are 

 termed compound or articulate : they may be branched or un- 

 branched ; they may be filamentous or scaly. They subserve 

 various functions, being protective, secretory, or absorbent. 



Various terms are used to describe hairs : filamentous hairs which are secre- 

 tory have frequently a dilated apex, and are termed capitate ; flattened hairs 

 which are star-shaped, are termed stellate ; discoid flattened hairs are termed 

 radiate or peltate ; the erect flattened hairs of Ferns are termed palcce or 

 ramenta. When hairs are stiff they are termed bristle* or seta. 



Special terms are used to indicate the nature and the distribution of the hairs 

 on a memher. A surface which bears no hairs is eaid to be glabrous ; when the 

 hairs are scattered the surface is pilose ; when the liairs are close and short, 

 villous ; when they are longer, tomentose. When the hairs are rather stiff, the 

 surface is hirsute ; when bristly, hiapid. When the hairs are borne on the 



