6o CELLULAR TISSUE. 



I. Hairs, elongated cells or rows of cells, simple or branched. Free ends not much 

 widened, or tapering conically : Filiform and conical hairs ; or widened into a head, 

 Capitate hairs {Pili cafitati). In the latter case the head is often articulated as a cell- 

 surface, or cell-body; these are transitions to the leading forms II and III, and may be 

 named according to convenience. 



I. Filiform and conical hairs. 



{a) Unicellular and unbranched forms belonging to this group arise, by the arching 

 outwards of the whole or part of the outer wall of one epidermal cell, so as to form a 

 cylindrical or conical protuberance above the neighbouring surface. The whole hair is a 

 single cell, of which a sac-like part of variable size protrudes as the body, the rest is 

 embedded in the epidermis as the foot. 



To this type belong all root-hairs. They are as a rule partial protuberances of the 

 outer wall of one epidermal cell ; when freely developed they are bluntly cylindrical, but 

 by application to the solid particles of soil they assume irregular forms and curvatures ', 

 rarely they are branched (in Brassica Napus observed by Sachs, Textbook, 2nd Eng. Ed. 

 p. 100), or they may arise in pairs from one epidermal cell. Only in Lycopodium^ can special 

 hair-cells be distinguished from the other epidermal cells on the root. From many of 

 the original similar polyhedral cells, a part of the lower end is cut off by an oblique 

 wall as a small cell, which divides further into 2-4 cells : each of these grows out into 

 a hair : the hairs therefore in the mature root are arranged in groups between the 

 elongated epidermal cells. 



On the foliage-leaf are to be found innumerable further examples. As a remarkable 

 form may be mentioned the conical hairs of many Borragineae, Loasese (Fig. 21 5) 

 HydrophyllesE (Wigandia), Urticese, many Cruciferse, Biscutella, Draba aizoides, Sinapis, 

 Brassica spec), also of latropha urens, and napseifolia. In the stronger forms of this 

 category, whether they sting or not, the base of the, conical hair is swollen, and 

 encroaches on the surrounding tissues. It is usually borne on a more or less protuberant 

 emergence, and is surrounded by a rosette of peculiarly formed subsidiary cells. To 

 certain of these hairs (Loasa, Nettles, latropha spec), which are characterised as a rule 

 by a button-shaped rounding-off of the upper end, and by the nature of their walls and 

 contents (Sect. 13), but by no further anatomical peculiarities of the hair itself or its 

 surroundings, the name stinging hairs (Stimuli) has been given. Compare the figures of 

 Meyen (Secretions-organe), Weiss, Martinet, and Rauter, and the more or less successful 

 figures of the stinging-hairs of the nettle in most text-books. Forms of this nature are 

 especially various in the Loaseae (Loasa bryoniaefolia, Cajophora lateritia). On the leaf 

 and^he carpels of the latter (Meyen I.e. Tab. VIII, B in our Figure 21) are seated 

 two sorts of conical hairs borne by slight emergences, and with their swollen bases 

 surrounded by subsidiary cells, (i) long, smooth, blunt, stinging hairs (a), and (2) shorter 

 ones, having the point oblique to the surface, with a thicker wall and numerous whorls of 

 short points turned upwards {b) ; further (3) small thin hairs, with a circle of reflexed 

 spicules at the blunt end, and many such laterally, these have a tapering base inserted in 

 the epidermis (c) : lastly (4) small 2- to many-celled capitate hairs U). 



The term unicellular branched hair may be applied to those described second in 

 Cajophora (Fig. 21, i, also c), inasmuch as the spicules or little hooks are short branches. 

 Transitions from the unbranched to the branched form are to be found in the Cruciferae- 

 in Draba aizoides, D. hispanica, Boiss., side by side with the above-named simple conical 

 hairs, occur others which, though otherwise of like character, are once branched at an 

 acute angle. More richly branched hairs, with many modifications and complications 

 are the prevailing form for the leaf of most Cruciferae, though they are not exclusively 

 present. The body of these unicellular branched hairs rises from the expanded foot 

 After a short distance, through which it remains undivided, it splits into 2-4 equal 



' Sachs, Exp. Physiol, p. 186. 



' Nageli und Leitgeb, Bau und Wachsthum derWurzeln, p. 124. 



