DEFENSIVE EQUIPMENT OF PLANTS 143 



almonds, hemp-seed, etc. (see p. 148); but in some the oil 

 is poisonous or strongly purgative — e.g., castor-oil seed, 

 croton seed, etc. 



(e) Volatile Oils. — These give to plants their scent. In 

 flowers they act as an attraction to the pollinating insects, 

 but in many cases the whole plant is strongly scented — 

 e.g., mint, lavender, sage. Camphor is an aromatic 

 crystalline substance distilled from the wood of the 

 camphor-tree ; it is poisonous. 



(/) Calcium Oxalate.^ — This salt is poisonous, but it is 

 produced naturally in the economy of plants, as a by- 

 product in the assimilation of proteins. It is generally 

 deposited in the form of hard gritty crystals within the 

 tissues (rhubarb-roots). In combination with potash, 

 oxalic acid occurs in sorrel. Crystals of calcium oxalate 

 are specially abundant in the outer leaves of some bulbs, 

 where they certainly serve a protective purpose. On one 

 occasion at Kew the outer leaves were stripped off the 

 bulbs of the Roman hyacinth ; but the bulbs, thus 

 deprived- of their protective armour, were found to be 

 quickly destroyed by snaUs. 



{g) Free Acids. — These occur very commonly in unripe 

 fruits, making them sour and uneatable. Citric acid is 

 abundant in the lemon, but in smaller quantities it is 

 present in many of our commonest fruits^ — e.g., cranberry, 

 cherry, and rose-hips. Mixed with an equal proportion of 

 malic acid, it is found in raspberries, red and black cur- 

 rants, bilberries, and -wild strawberries. Unripe apples 

 and pears contain malic acid only. As the starch of the 

 unripe fruits becomes converted into sugar, these acdds 

 gradually disappear, and the fruits become sweet and 

 palatable. 



[h) Alkaloid Poisons. — The most deadly organic poisons 

 known to the chemist are found in plants. Digitalin is 

 obtained from the leaves of the foxglove [Digitalis pur- 

 purea) ; strychnin from the seeds of Strychnos Nux-vomica ; 

 belladonnin and atropin, from the deadly nightshade 

 {Atro"pa Belladonna) ; aconitin from the root of Aooni- 

 tum Napellus ; morphia from the juice obtained from 

 the unripe capsules of the opium-poppy (Papaver somni- 

 ferum). But in most of these cases the whole plant is 

 more or less poisonous, and this may extend even to the 

 flowers and fruits. In the aconite the very honey secreted 

 in the flower is poisonous. 



