320 PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 



of the leaf to which they belong; they bend down nearly at a right angle almost 



immediately above their place of insertion, so that the long tapering part of the hair 



cell lies on the leaf -surf ace, as shown in fig. 77 3 . When such hair-forms, in great 



numbers and parallel to one another, entirely cover the surface of the leaf, light is 



strongly reflected from them, and the surface looks just like a piece of silk. Such 



a covering of hair, which is seen particularly well on the shining foliage of the South 



European bindweeds (Convolvulus Gneorum,' nitidus, oleaefolius, tenuissimus, &c), 



is termed " silky " (sericeus). Two varieties of this may be distinguished, via. the 



more usual case in which all the hairs of the leaf lie parallel with the midrib, and 



the rarer case where the hairs assume a different position on the right and left of 



the midrib, the whole of those on either side being respectively parallel to the 



lateral ribs of their respective sides. The reflected light then only meets the eye of 



the observer, in any one position, from one half of the leaf, the other half therefore 



appearing dull. In such a case the whole leaf has that peculiar shimmer, changing 



on the slightest movement, which we admire on the wings of certain butterflies, and 



which is also shown by that variety of silken material known as satin. When the 



unicellular hairs do not lie on the surface, but rise up from it, the shimmer is 



altogether absent, or is only present to a small extent. If the hairs are short, very 



numerous, and closely pressed together, they are said to be " velvety " (holosericeus); 



if they are of greater length and situated further apart, the expression " shaggy " 



(villosus) is used. Hairs which consist of single elongated air-containing cells, 



much twisted and bent, with thin and pliant walls, are called wool-hairs, and the 



covering formed by them is said to be " woolly " or " tomentose " (lanuginosus). 



Woolly hairs are always twisted spirally, sometimes loosely, sometimes tightly — 



frequently almost like a corkscrew. As a rule the spiral is in the opposite direction 



to the movement of the hand of a watch, whose direction is said to be to the left. 



It should also be noticed whether the elongated twisted cells of the wool-hairs are 



circular in cross section, as in the South European Centaurea Ragusina (see fig. 77 6 ), 



or whether they are compressed like a ribbon, as in Ghnaphalium tomentoswm 



(fig. 77 4 ). The latter case is by far the most common. 



Multicellular clothing hairs originate by the repeated division of certain 

 epidermal cells caused by the formation of separation walls. These dividing walls 

 are either all parallel to the surface of the leaf or stem, or some of them are perpen- 

 dicular to the plane of the leaf. In the first case the cells are usually arranged like 

 the links of a chain, and are termed jointed or articulated hairs. When such arti- 

 culated hairs are short and not interwoven — as, for example, is the case in the 

 beautiful gloxinias (see fig. 77 2 ), the surfaces clothed with them appear like velvet; 

 when they are elongated, curved, and twisted and entwined, the leaf appears to be 

 covered with wool (see fig. II 1 ), and to the naked eye this form of covering is the 

 same as that already stated to be shown by unicellular covering hairs. Silky coats 

 are also produced by multicellular hairs, even by such a peculiar form as is repre- 

 sented in fig. 78 3 . These hairs are developed in the following manner. A super- 

 ficial cell by the formation of a septum parallel to the leaf -surf ace divides into two 



