46 PART I. MORPHOLOGY. [ 14 



cans), which are developed on the ventral surface of the dorsiven- 

 tral stem, and adhere closely to the tree- trunk or wall on which 

 the plant is climbing. 



In some rare cases the aerial root is a tendril, as in Vanilla 

 aromatica, Lycopodium rupestre and other species, and in some 

 Melastomacefe (Mcdinilla radicans, Dissochseta). 



Roots are occasionally developed as thorns, as in the Palms 

 Acanthoriza and Iriartea, and in Myrmecodia (Rubiaeese). 



In some species of Jussisea (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. 



(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 unbranched ; they may be filamentous or scaly. They 

 subserve various functions, being protective, secretory, or ab- 

 sorbent. 



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

 secretory 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 

 palece or rametita. When hairs are stiff they are termed bristles or 

 setce. 



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

 hairs on a member. A surface which bears no hairs is said to be glabrous ; 

 when the hairs are scattered the surface is pilose; when the hairs are 

 close and short, villous ; when they are longer, tomentose. When the hairs 

 are rather stiff, the surface is hirsute ; when bristly, hispid. When the 

 hairs are borne on the margin only, the member is said to be ciliate. A 

 surface with closely appressed hairs islepidote ; a member bearing ramenta 

 is ramentaceous. 



The root-hairs demand special notice. Root-hairs are hairs which 

 perform the functions of absorption and attachment ; they are com- 

 monly developed on roots, though not always, for they are absent 



