MORPHOLOGY 109 



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 de- 

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

 mucilaginous modification of their cell walls. Where 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 ffippocastanum), 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. 119) act in a similar manner. The emergences on the 

 leaves of the Sundew (Drosera), described as digestive glands (Fig. 120), discharge 

 glistening drops of mucilaginous matter, not under the cuticle, but from the free 

 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 also often excrete sugary solutions directly from their surfaces. In 

 flowers these serve to attract insects, which effect pollination, while on other parts 

 of the plants they are known in certain cases to attract ants, which protect 

 the plant. The osmotically active substances in the nectar are in the first 

 instance derived by transformation of 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 thus leads to the continued formation of the 

 nectar. 



In some of the Piperaceae and Begoniaceae, and in some species of Ficus, the 

 epidermis is composed of several layers ; but this is a 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. 76) 

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

 of Ficus elastica, already referred to (p. 63), occur in single swollen epidermal cells. 

 A many-layered root epidermis is also met with, as in species of Asparagus, Crinum 

 and Lycoris ( 107 ). The many - 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. 48), a parchment-like sheath surrounding the 

 roots, and often attaining a considerable 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 is filled with water, 

 the underlying tissues impart a greenish tint to the root ; but if it contains only 

 air the root appears white. The epidermis of fruits, and particularly of seeds, 

 exhibits a considerable variety of modifications in its mode of thickening, and in 

 the relations the thickening layers bear to one another. The purpose of these 

 modifications in the epidermis becomes at once evident, when it is taken into 

 consideration that, in the case of fruits and seeds, in addition to protecting and 

 enclosing the internal parts, the epidermis has often to provide for their 

 dissemination and permanent lodgment. 



The Vascular Bundle System. The PRIMARY VASCULAR BUNDLES 

 extend in the form of strands throughout the body of the higher 

 plants. In more transparent stems, such as those of Impatiens parvi- 

 flora, the bundles may be clearly distinguished and their course 

 followed. The arrangement of the bundles of leaves is apparent from 



