90 PLANT-LIFE ON LAND [CH. 



Rue and the Sweet Rush (Acorus) by ethereal oils : 

 others again are protected by sharp needle-shaped 

 crystals embedded in their tissues. These, which are 

 especially effective against the attacks of snails, are 

 found in many Monocotyledons, such as Narcissus or 

 Orchis, and in some Dicotyledons, such as the Vine. 

 The sharp crystals are usually associated with mucil- 

 age, which swells on access of water when the tissues 

 are crushed or cut by chewing or biting. The crystals 

 are thus dislodged. Pointing irregularly in all direc- 

 tions, and puncturing the delicate tissues of the 

 mouth, they make the plant distasteful, and thus 

 protect it from further attack. The experiments of 

 Professor Stahl with snails have shown how effective 

 this protection actually is, even when the snails had 

 been famished for the purpose of the test. In other 

 cases the protection may be by pointed hairs, spines 

 or thorns, and this is particularly prevalent in dry 

 climates, where the succulent character of the vegeta- 

 tion would offer peculiar attractions to animals. The 

 most conspicuous examples are among the Cactaceae. 



These notes must suffice to suggest some of the 

 many ways in which plants, owing to relatively minor 

 characters which they possess, are enabled to subsist 

 even where they are exposed to the attacks of 

 herbivorous animals. 



Similarly they have to meet the risks of climate. 

 The mobile animal may protect itself against these 



