56 



BOTANY 



PART I 



to diminish the transpiration and are a protection against direct 

 sunlight. The root-hairs (Figs. 51 J5, 158 r) are tubular prolongations 

 of living cells of the epidermis of the root and serve for absorption 

 of water. Very diverse substances are excreted by glandular hairs 

 (Figs. 75, 76, 77). 



Certain hairs with abundant protoplasm and peculiar structure serve to receive 

 mechanical stimuli according to G. HABERLANDT ( 42 ). They occur on stamens, 

 petals, and the joints of leaves, and are known as tactile papillae, hairs, or bristles. 



4. EMERGENCES, unlike hairs, are not formed solely by epidermal 

 cells, but a number of cells, lying more or less deeply in the sub- 

 epidermal tissues, also take part in their formation. They are some- 

 times glandular, and in other cases 

 serve as organs of attachment. 



Thus, for example, only a few rows 

 of sub -epidermal cells enter into the 

 formation of the emergences (Fig. 56) on 



FIG. 54. Scale-hair from the lower side of the leaf of Shepherdia canadensis. A, Surface view. 

 B, Longitudinal section, (x 240. After STBASBURGER.) 



the margins of the stipules of the Pansy (Viola tricolor], which are glandular. 

 Deeper-lying tissue takes part in the construction of the anchor-shaped attaching 

 organs, over 1 mm. long, which clothe the fruit of the Houndstongue (Cynoglossum) 

 and lead to its dispersal by means of animals. The prickles of the Rose or 

 Bramble are still larger emergences that are of assistance in climbing. 



(b) Boundary Tissue formed of Corky Cells. In many cases, 

 and especially when the epidermis does not remain alive and 

 functional during the life of the organ which it covers, the tissues 

 of the body become limited and protected even more efficiently by 

 suberised cells. Such cells also in the form of layers or sheaths serve 

 to bound and delimit certain living masses of tissue from others within 

 the plant body. Their origin may be primary or secondary. The 

 suberisation is brought about by suberised lamellae being deposited 

 on the pre-existing wall, while other layers of the wall frequently 

 become lignified. Three kinds of suberised boundary tissues can be 

 recognised: (1) The cutis tissue; (2) the endodermis; (3) the cork, r 



