1881.J 



MICKOSCOPICAL JOUKNAL. 



103 



The Epidermal Organs of 

 Plants. 



BY CHARLES F. COX, F.R.M.S. 



It is, of course, well known that 

 those parts of plants which are ex- 

 posed to external influences — to light, 

 heat, evaporation, etc. — become com- 

 pacted and hardened into a rather 

 dense and rigid tissue called the epi- 

 dermis. This differentiation is not 

 well marked in roots and submerged 

 parts of aquatic plants, but even in 

 such parts we recognize at least a 

 theoretical difference between the 

 most exterior layers of cells and those 

 of the so-called parenchyma lying 

 beneath. 



Upon the aerial portions of plants 

 there is generally a tissue external 

 to the epidermis — of the nature of 

 a very thin, usually homogeneous 

 and transparent pellicle or skin, — 

 termed the cuticle. 



The epidermis, however, may be 

 regarded, physiologically, as the most 

 external organized and organ-produc- 

 ing substance of the plant. 



From this epidermis are developed 

 all external appendages, known as 

 hairs, villi, papillae, tentacles, bris- 

 tles, thorns, scales and glands, with 

 analogous and homologous organs, 

 including probably the sporangia of 

 ferns, and even the ovules of phane- 

 rogams. 



It has been proposed to class all 

 such lateral growths from the epi- 

 dermis under a single name : Trich- 

 omes. This grouping, however, not 

 being entirely satisfactory in all 

 respects, is merely tentative ; and 

 there is good reason to think 

 that, when microscopical botany has 

 been thoroughly worked up, the 

 whole theory of the epidermis and its 

 appendages will have to be reconsider- 

 ed and much modified. 



Hairs are the most attractive of 

 the epidermal appendages, and are 

 exceedingly varied in form, as well 

 as often charmingly . beautiful in 

 appearance. They have long been 

 favorite objects of observation with 



microscopists, though I cannot say 

 that they have long been objects of 

 scientific study and investigation; for, 

 as far as I am aware, they have sel- 

 dom attracted attention except for 

 their external symmetry and beauty. 



This mere prettiness of plant-hairs 

 cannot be adequately expressed in 

 description ; and, so far from won- 

 dering that it has attracted attention, 

 I am surprised that it has not attract- 

 ed more ; for I am sure that no class 

 of microscopic objects offers more 

 various beauties of outline, ornamen- 

 tation and color, than do these ex- 

 quisite gems of the vegetable world. 



The outlines of different kinds 

 of plant-hairs have often been figured, 

 and I will not consume time and 

 space in any attempts at an extended 

 description here. It may be well, 

 however, to mention the more char- 

 acteristic forms. 



To begin with, we have what may 

 be called the right hair, which is a 

 simple, perpendicular, or more or less 

 appressed, unicellular growth, such as 

 may be found on Lithospermum hir- 

 tum or Lithospermum canescens. 



Jointed hairs are similar in outline 

 to the right hairs ; but instead of be- 

 ing unicellular, they are composed of 

 a series of cells or nodes. Such 

 hairs are found upon many of the 

 Compositae, — for instance, Pluchea 

 bifrons. 



In segmented hairs the component 

 cells are separated by deeper con- 

 strictions than in jointed hairs ; and 

 they resemble, when filled with their 

 living fluids, strings of translucent 

 beads. Such hairs grow upon the 

 stamens of Tradescantia Virginica 

 and upon the leaves of Polemonium 

 cceruleum. 



Cleft hairs resemble right hairs 

 which have been split part way down, 

 and the separated portions of which 

 have become more or less depressed. 

 In some cases this depression pro- 

 ceeds so far that the hair appears to 

 be a long, doubly terminated, hori- 

 zontal cell attached to the leaf by a 

 pedicel at its centre. Such hairs 



