462 METABOLISM IN LIVING PLANTS. 



the flowers in order to obtain the honey, but which would not be welcome guests 

 to the plant. 



Fats take a part in the life of plants similar to that played by ethereal oils. 

 Fats are combinations of fatty acids with glycerine, and may be divided into two 

 groups; in one group the members dry up when exposed to the air by the separation 

 of carbonic acid, as, for example, in poppy oil and linseed oil, which are used for this 

 very reason in oil painting. In the other group, e.g. in almond and olive oils, the 

 members remain fluid when exposed to the air, and give rise to malodorous fatty 

 acids, and when this change occurs the body is said to become rancid. The most 

 abundant production of fats takes place in fruits, seeds, and spores, where they are 

 stored up as reserve materials, but in many instances they also function as attrac- 

 tive or protective agents. Nor must we forget the crystalline or amorphous fatty 

 excretions formed on the epidermis of foliage-leaves, stems, and fruits, which are 

 popularly known as " bloom ". These are very like wax, and have a very manifold 

 significance; they prevent hurtful moistening by water, regulate transpiration 

 under certain circumstances, and can also ward off the disadvantageous attacks of 

 animals. The branches of many willows which bear honey -laden flower-catkins, as, 

 for example, those of Salix pruinosa and daphnoides, are provided with these wax- 

 like, extremely smooth and slippery coverings up which the unwelcome wingless 

 ants, scenting the honey in the catkins, in vain try to climb. 



Alkaloids and glucosides are developed principally as means for protecting 

 the green tissues of the leaves and fruit, and the underground portions of the 

 plants — the roots, rhizomes, tubers, and bulbs — against demolition and extinction 

 by animals. The alkaloids are distinguished by the presence of nitrogen in 

 them. Some of them contain no oxygen, and are volatile, as, for example, 

 trimethylamine, which occurs in the leaves of many oraches, and in the flowers 

 of Hawthorn and Pear, as well as in the American Pachysandra. Most, however, 

 are non-volatile, and contain oxygen. To this latter class belong the well-known 

 alkaloids — morphine, nicotine, atropine, and strychnine, which are poisonous 

 to man and most mammals; also the well-known drugs — quinine, cocain, and 

 many others. Leaves provided with these materials are rejected as food by 

 grazing animals, and consequently they at least may be regarded as efiicient in 

 protecting the plants from being devoured. Only the volatile trimethylamine in 

 flowers can serve to attract insects. Glucosides, of which more than a hundred 

 are already known, have a use very similar to that of the alkaloids. Saponin 

 is poisonous to man and mammals; amygdalin splits up into the poisonous prussic 

 acid, oil of bitter almonds and sugar; and many others behave in exactly the 

 same way. Tannin has an extremely bitter taste, and therefore protects branches, 

 cortex, and fruits from being eaten. It is interesting to see, that in many fruits 

 which are distributed by means of animals, the pericarp remains acid and unwhole- 

 some in consequence of bitter or poisonous glucosides, until the seeds hidden 

 within have matured. As soon as they can germinate, the glucosides become 

 changed; they are split up by means of ferments, which will be described later, 



