ELEMEKTART STRUCTURE. 



49 



Fiq. 116. 



form prickles, which are defined as large hardened processes 

 terminating in a sharp point, and springing from the epidermis 

 or the bark of plants {fig. 115). They are especially abundant 

 on the stems of the Rose and Bramble. These must be dis- 

 tinguished from spines, to be hereafter described when speaking 

 of branches. (See p. 109.) 



The hairs above described commonly contain 

 fluid of a watery nature, which may be colour- 

 less or coloured. In other instances they are 

 filled with various special secretions usually 

 of an oily or resinous nature, under which 

 circumstances they are termed ylaiidular hairs, 

 and will be again mentioned under the head 

 of glands, to which they properly belong. The 

 other* kinds of hairs, namely those without 

 secretions, have been called by Lindley and 

 others, lymphatic hairs. 



Hairs occur upon various parts of plants, 

 and, according to their abundance and na- 

 ture, they give varying appearances to them, 

 all" of Avhich are distinguislied in practical bo- 

 tany by special names. The more common 

 position of hairs is upon the leaves and stems, 

 but they also occur on the parts of the flower, 

 ■the fruit, and the seed. The substance called 

 cowhage or cowitch is the hair covering tlie 

 legumes of Mucuna pruriens, while cotton is 

 the hair covering the seeds of various species 

 of Gossypium. Cotton may be readily dis- 

 tinguished under the microscope from the 

 various kinds of liber-cells already described 

 (see page 30), from the circumstance of its component cells 

 collapsing when dry from not possessing any stiff thickening 

 layers, so that it then resembles a flat band, more or less twisted, 

 with thickened edges {fig. 116, a); while liber-cells from con- 

 taining thickening material in their interior, always maintain 

 their original cylindrical forms and tapering extremities {fig. 

 116, b). On young roots we tind cells prolonged beyond the 

 surface which are clearly of the nature of hairs, and have 

 accordingly been termed radical hairs {fig. 232). Thehairs which 

 occur on the parts of the flower frequently serve an indirect 

 part in the process of fertilization, by collecting the pollen or 

 fructifying powder which falls from the stamens; hence such 

 are termed co//ec<i«(/ hairs {fig. 117). A peculiar kind of col- 

 lecting hair occurs on the style of the Campanulas or hare-bells, 

 the upper part, at the period of fecundation, being retracted 

 within the lower (j?5r. 118). In some cases we find different 

 parts of the plant becoming transformed into hairs. Thus in 



Fig. 116. o. Cotton. 

 h. Flax fibres. 



