HAIRS ETC . 
114 
often doubtful. The dense hairy covering of many xerophytes checks 
transpiration, the stinging hairs of Urticaceae, Loasaceae, &c. are pro- 
tective, the barbed hairs of the fruits of Galium, Blumenbachia, &c. serve 
for animal distribution, and so on. 
Emergences are outgrowths of the surface which arise from other 
tissues as well as the epidermis, e.g. the tentacles of Drosera, which 
contain vascular bundles, the prickles of Rosa and Ribes, &c. 
Latex is a milky fluid, usually white or yellow, contained in special 
laticiferous vessels or cells which run through the tissues of certain plants, 
e.g. Euphorbia, Cichorieae (Compositae), many Papaveraceae, Apocyna- 
ceae, Asclepiadaceae, Sapotaceae, &c. The fluid contains substances 
of various kinds ; some are of use in nutrition, and to some extent the 
laticiferous tissue therefore replaces the phloem ; others, e.g. caoutchouc, 
are apparently excretory products of little or no use to the plant. 
Raphides are needle-shaped crystals of calcium oxalate, contained in 
the cells, especially in young growing parts and in Monocotyledons. The 
painful effects of chewing a piece of the leaf of Arum maculatum are 
due to the raphides sticking into the mucous membrane. 
Water-pores or hydathodes are openings like stomata upon leaves or 
elsewhere, through which the plant excretes water, often containing 
other substances, e.g. chalk, in solution. During the day evaporation 
carries off the water, but at night it often accumulates and is mistaken 
for dew. The drops of water so commonly seen in the morning on the 
tips of grass blades have been thus formed. Water-pores are also found 
in many species of Saxifraga, Tropaeolum, Fuchsia, Caladium, &c. 
When the water contains sugar in solution, a nectary (p. 88) is formed 1 . 
These are usually in flowers, but there are many cases of extra-floral 
nectaries, e.g. on the leaves of the cherry-laurel (Prunus), stipules of 
Vicia, Viola, &c. Extra-floral nectaries usually attract ants and this 
brings to the plant a certain advantage by keeping off caterpillars, &c. 
Several tropical plants show extreme cases of adaptation in this direction, 
by housing and feeding standing armies of fighting ants. Such plants 
are said to be myrmecophilous (see Acacia, Cecropia, &c.). In this 
connection mention may be made of the honey-dew so common on leaves 
(see Acer, Pithecolobium, &c.), and of the domatia or little dwellings 
inhabited by mites, usually constructed of hairs, or sometimes in little 
hollows or grooves of the leaf (see Fraxinus, Anamirta). 
Resin-passages are large canal-like intercellular spaces in the tissues of 
certain plants, e.g. Coniferae. Into them the plant secretes resin ; this 
appears to be a waste-product, though it may have an incidental value 
in protecting the wood from decay, or in other ways. 
Oil-cavities containing oils of various sorts, are frequent in various 
plants, especially in the leaves, where they show as translucent dots when 
held up to the light. They occur in Rutaceae, Guttiferae, &c. 
Glands are organs secreting fluids upon the surface of a plant (or 
sometimes internally). Water-pores and nectaries may be included under 
this head, and other glands secrete oils, &c. 
1 Usually there is no pore in a nectary, the fluid being excreted by 
the superficial cells. 
