SECT. I 



.MORPHOLOGY 



and in special cases, as in the scale hairs of Ferns (Fig. 113), they may 

 even have the shape of a small leaf. 



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. Thus, for example, while 

 only a few rows of sub-epidermal cells enter into the formation of the 

 emergences (Fig. 114) on the margins of the stipules of the Pansy 

 ( Viola tricolor), much deeper-lying tissue participates in the development 

 of the emergences which, as prickles, serve in the case of roses as a 

 means of protection, and at the same time are of assistance in climbing. 

 Vascular bundles also may be included within the emergences, as is well 

 shown in the club-shaped digestive glands or tentacles (Fig. 115) on 

 the leaves of the Sundew (Drosera). Some emer- 

 gences resemble in structure certain of the meta- 

 morphosed members of the plant body described 

 in the preceding chapter ; the resemblance be- 

 tween prickles and thorns, for instance, is parti- 

 cularly noticeable. The phylogenetic origin of 

 emergences, and therefore the morphological value 

 of metamorphosed members, is altogether different. 

 The irregular distribution of emergences affords 

 an easy means of distinguishing them from such 

 metamorphosed members as resemble them in 

 appearance. , 



Both hairs and emergences sometimes 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 (cf. 

 p. 91). Hairs which function as Irydathodes are 

 usually multicellular; they are provided with 

 a short stalk and terminate in a head. Other 

 glandular hairs excrete a resinous substance. The 

 hairs of Primula sinensis (Fig. Ill) are in reality 

 such glands, and it is from their excretions that 

 the plant derives its peculiar odour. The cuticle 

 of the terminal globular head is pressed away fig. 115.— Digestive gland 

 from the cell wall by the resinous matter ex- flom Hmsem mtundi- 

 creted from the hair, until, finally, the bulging 

 cuticle is ruptured and the resinous secretion exudes. The similar 

 but more complicated glandular hairs of Hops (Fig. 112) 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 pressed out from 

 the underlying cell wall as a continuous membrane (Fig. 112, B). 



