SECT. I MORPHOLOGY 11'.) 



resemblance between prickles and thorns, and between haptera and 

 lateral roots will serve as examples. They are not, however, to 

 be traced back in origin to such members (p. 51). 



Both hairs and emergences frequently act as secreting organs, and 

 are then termed GLANDS. In many cases they are concerned with the 

 active exudation, and at times also the absorption of water. They 

 then belong to the class of organs designated hydathodes (^^^) by 

 Haberlandt. Other glandular haii's excrete a resinous substance. 

 The hairs of Primula sinensis (Fig. 127) are in reality such glands, in 

 Avhich the cuticle of the terminal globular head is pressed away from 

 the cell wall by the resinous matter excreted from the hair, until 

 finally the bulging cuticle is ruptured and the resinous secretion 

 exudes. The hairs of this and other species of Primula (especially 

 P. ohconica) are capable of exciting inflammation in the skin of those 

 handling them (^•^■'). Only some persons are susceptible to the eftect. 

 The similar but more complicated glandular hairs of Hops (Fig. 128) 

 produce a secretion called lupulin, to which beer owes its bitter taste 

 and distinctive aroma. The secretion is set free by the bursting of 

 the cuticle, the latter having been previously raised up from the 

 underlying cell wall as a continuous membrane (Fig. 128 B). Hairs 

 and emergences with abundant protoplasmic contents occur on irritable 

 stamens, perianth leaves, and pulvini, and, as Haberlandt (^~^) showed, 

 act as tactile papillae, hairs, or bristles in the reception of stimuli. 



The mucilaginous matter produced in young buds by the mucilage papillae or 

 COLLETERS lesults from the partial dissolution of the cell wall under the cuticle. 

 After the mucilaginous secretion has been discharged by the ultimate rupture of 

 the cuticle, another new cuticle forms over the cell wall, and the process is again 

 repeated. The colleters are special forms of hairy structures, and are often 

 developed in buds to protect the young organs from drying, by means of the 

 mucilaginous modification of their cell walls. Wliere the dissolution of the cell 

 wall is accompanied by secretions from the underlying cells, the colleters assume 

 rather the character of glandular hairs. Such glandular colleters are common 

 in the winter buds of trees ; in the Horse-chestnut {Aesculus Hippocastanum), for 

 example, the bud-scales of the winter buds are stuck together by a mixture of 

 gum and resin, which has exuded from colleters of this nature. The glandular 

 hairs of the Pansy (Fig. 129) act in_a similar manner. The emergences on the leaves 

 of the Sundew {Droscra), just mentioned as digestive glands (Fig. 130), discharge 

 glistening drops of mucilaginous matter, which accumulate on tlie free surface 

 of the glands at the ends of the tentacles, through the porose cuticle. Small 

 animals are caught by means of these sticky excretions, and are afterwards digested 

 by the plant. The nectaries also often excrete sugary solutions directly I'rom 

 their surfaces. In flowers these serve to attract insects, which effect pollination, 

 while on other parts of the plant they are known in certain cases to attract ants, 

 which protect the iJant. The osmotically active substances in the nectar are in 

 the first instance derived by transformation ( f the outer cell walls, or are secreted 

 by the cells. The presence of these substances on the surface of the nectary 

 attracts water from the tissue beneath, and tlius leads to the continued formation 

 of the nectar. i 



