100 BOTANY 



The mucilaginous matter produced in young buds by the mucus 

 papillae or colleters results 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 continually developing cell wall, and the process is again repeated. 

 The colleters are but 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. Where the dis- 

 solution of the cell wall is accompanied by secretions from the under- 

 lying cells, the colleters assume rather the character of glandular 

 hairs. Such glandular colleters are prevalent in the winter buds 

 of trees ; in the Horse-chestnut (Aesculus Hvppocastanum), for example, 

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

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

 glandular hairs of the Pansy (Fig. 114) act in a similar manner. 



The emergences on the leaves of the Sundew (Drosera), described 

 as digestive glands (Fig. 115), discharge glistening drops of muci- 

 laginous matter, not under the cuticle, but directly from the surface of 

 the glands at the ends of the tentacles. Small animals are caught 

 by means of these sticky excretions, and are afterwards digested 

 by the plant. The nectaries of flowers also often excrete sugary 

 solutions directly from their surfaces. The excreting cells are generally 

 thin-walled and not cuticularised. The excretion of nectar may also take 

 place through water-stomata. Finally, intercellular secretion 

 cavities are found on the emergences of the inflorescence of 

 Dictamnus Fraxinella. The lower distended portions of its ampullaceous 

 emergences contain a fragrant ethereal oil, which fills the lysigenous 

 cavity formed by the dissolution of the secreting cells (Fig. 116). 

 In addition to these glandular emergences, other internal glands which 

 are developed from the epidermis and the underlying layer of cells 

 are found in the leaves of Dictamnus. 



In some plants the epidermis is composed of several layers ; but 

 this is of comparatively rare occurrence. Such a many-layered 

 epidermis results from a division of the young epidermal cells parallel 

 to their external surface. The epidermis of Ficus elastica (Fig. 88) 

 has three layers, and serves as a reservoir for accumulating water. 

 The cystoliths of Ficus elastica, already referred to, occur in consider- 

 ably swollen epidermal cells. The multi -layered epidermis of the 

 aerial roots of many Orchids, and of various Aroids, undergoes a peculiar 

 modification and forms the so-called VELAMEN radicum (p. 42), a parch- 

 ment-like sheath surrounding the roots, and often attaining a consider- 

 able thickness. The cells of this enveloping sheath are generally provided 

 with spiral or reticulate thickenings, and lose their living contents. 

 They then become filled with either water or air, depending upon the 

 amount of moisture contained in the surrounding atmosphere. These 

 root-envelopes absorb water like blotting-paper ; when the velamen 



