58 CELLVLAR TISSUE. 



refer for the complete enumeration of them to the works quoted, especially those of 

 Weiss and Martinet. 



In face of the various facts and opinions it is our first business to determine what one 

 understands by hair-structures or Trichomes. There are two opposed opinions on this 

 point. The advocates of the one apply this name only to outgrowths belonging to and 

 derived from the epidermis — in the sense indicated in Sect, i — ; others apply it to all 

 characteristically formed outgrowths of the plant, to which the conceptions or traditional 

 terms stem, leaf, root cannot be applied, whether these protuberances belong to the 

 epidermis alone, or whether the subepidermal cells, and even the vascular system take 

 part in their formation, e. g. the prickles of the Roses, of species of Smilax and Solanum, 

 and of the Thorn-apple, &c. The foundation of the latter view seem^ to me to lie less 

 in observable facts, than in the historical fact that outgrowths such as prickles and warts 

 were formerly included among hair-structures, since it was thought that they belonged 

 to the Epidermis \ If we deviate from this view, which is now proved to be incorrect 

 for the majority of cases, the term trichome must also be restricted, and all outgrovrths 

 must be excluded from it, which include in themselves more than epidermis. Otherwise 

 a quite unnecessary confusion would be brought into well-founded views and relations 

 since, if one includes among trichomes all outgrowths of the surface of stem or leaf, one 

 must also include those of the leaf margin, i. e. all leaf-teeth. If we adhere to the 

 anatomical and developmental facts which are clearly before us, we easily obtain the 

 definition here given of the idea of the hair-structure or Trichome as equivalent to an 

 outgrowth of the epidermis, and the distinction of this from those outgrowths in which 

 more than the epidermis takes part, for which the term Emergences proposed by Sachs 

 (Textbook, 2nd Eng. Ed. p. 161) is suitable. 



The distinction between hairs and peculiarly formed epidennal cells may present 

 difficulties in many single cases, e. g. in the genus Mesembryanthemum, where large cells 

 scattered in the epidermis bulge outwards in M. crystallinum as huge bladders, while in 

 other species they scarcely rise above the surface. But it is just the same whether one 

 calls them hairs or not. The case, described by Uhlworm (/. c, Fig. 28-30), for the warts 

 of Gunnera scabra, which are covered by a piece of epidermis consisting of cells elongated 

 perpendicular to the surface, may be denoted, as above, or one can speak of a group 

 of laterally-united prismatic unicellular hairs, or one may (with Uhlworm) term the whole 

 piece of epidermis a multicellular trichome, which in that case forms an exception from 

 the rule of the origin of each trichome from one initial cell. 



Starting, as is always necessary in defining types, from clearly characterised forms, 

 the above established leading types of hair-structure may easily be separated according 

 to their external development, and they are as a rule easily distinguished, without very 

 exact investigation, by habit and consistency. Their distinction is therefore to be 

 recommended for use in systematic Botany, which has as yet made use of these relations 

 less than they might be employed. Intermediate forms are by no means absent. But 

 these may easily be subordinated, or appended to the types. It is however often 

 indifferent to which of the types a special case is appended, and this is defined as 

 convenience may dictate. One may, for instance, call the flat horizontal appendages of 

 the Elseagneae, or of Polypodium Lingua, stellately branched, multicellular hairs, just as 

 well as stellate scales ; or a capitate hair with a large compound head may just as well be 

 termed a long-stalked scale or a dermal wart. 



Within the main limits, special forms are incredibly various as regards form, special 

 articulation, and direction, &c. The detailed description of them is the subject of 

 the most special systematic study, and their minute classification, though it might have 

 a significance at the times of Guettard and Schrank, can only be idle play at the 

 present time. Here, therefore, we may give only a few details and one or two drawings 



\ Compare e.g. Schleiden, Grundz. 3 Aufl. I. p. 271; Unger, Anat. und Physiol, p. 188. 



