56 CELLULAR TISSUE. 



be specially noticed here, some few families are distinguished by complete or almost 

 complete absence of hair-structures, as the Equiseta, the Coniferse, the Potamcse, and 

 Lemnace». 



They occur in the majority of genera and species, though certainly to a very 

 variable extent. 



Different vegetative adaptation does not determine the presence or absence of 

 hair- structures ; they occur under all states of adaptation, even in submerged species, 

 as Callitriche, Nympha^a, and species of Ranunculus. On the other hand, their 

 number and development seems certainly to be influenced by the nature of the 

 environment, since observation shows that in allied species, and in individuals of the 

 same species, the hairiness increases with the sun-light, dryness, and airiness of the 

 spot. But there is no safe foundation for a definite assertion on this point. 



As regards the distribution of single /bn/is of hairs through families and genera, 

 the case is similar to that of the forms of foliage leaves. On the one hand there is 

 great uniformity of the majority of species and genera of one family, at least as regards 

 07i€ characteristic form of hair, so that one may speak, for instance, of the bristles of 

 the Borraginese, the short (glandular) capitate hairs and scales of the Labiatee, the 

 stellate hairs of the Cruciferae, the tufted hairs of the Malvaceae, the multiseriate 

 shaggy hairs of the Melastomea^, the delicate branched hairs accompanying the 

 capitate hairs of the genus Lavendula, the three characteristic forms of hair of most 

 of the Hieracia, &c. On the other hand, in natural families (e. g. Compositse, 

 Labiatce), and even genera (e. g. Solanum), the most various forms exclude one 

 another ; or one characteristic definite hair-form recurs on corresponding parts in the 

 most remote genera and families, as the stinging hairs on the leaf of Urticaceae and 

 Loasese, the shield-like stellate hairs or scales on those of the Oleaceae, Elseagneae, 

 and species of Solanum, Croton, Bromeliace^e. and Ferns; the spindle-shaped, 

 appressed hairs, with central attachment of the IMalpighiacese and Cruciferae, &c. 



The development of hair-structures, both uni- and muldcellular, begins, in all 

 certainly investigated cases, from one epidermal cell, as hiitial cell. This cell protrudes 

 beyond the outer surface of those surrounding it : the part within this surface 

 develops into the foot, the protruded portion into the body of the hair. The growth 

 which ensues is, according to the special case, acropetal, basipetal, or intercalary, as 

 regards the hair itself (Rauter). It is obvious that in forms consisting of more than 

 one cell, divisions accompany growth, and the successive division-walls appear in 

 definite number and position for each case ; further, that the definite form and 

 articulation depend upon the successive divisions, and the growth of the cells after 

 the division is complete. In 2- to 4-seriate shaggy hairs, scales, &c., in which the 

 rows of cells are continuous into the foot, and are there represented by two or many 

 cells side by side in the epidermis, e.g. Hieracium aurantiacum and its allies, division 

 of the initial cell perpendicular to the epidermal surface begins almost simultaneously 

 with, or very soon after the protrusion of the body outwards. The development of 

 an emergence bearing a hair begins later than the origination of the hair itself by 

 local growth of the subepidermal meristem, and of the epidermal cells surrounding 

 the initial cell of the hair. 



The origination of the hair-structures begins on stem and leaf at a very early age, 

 on the former however, as a rule (but not always), not above tlie point of insertion of 



