CHAPTER IT. SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 711 



carnivorous. The case of Drosera may be selected for illustration. The upper 

 surface and the margin of a leaf of this plant bears numerous glandular appen- 

 dages, the tentacles (see Fig. 42, p. 66). The glands at the ends of the tentacles 

 continually excrete a viscid liquid. When an insect comes into contact with 

 one of the marginal glands, it sticks to it ; this stimulates the tentacle, and it 

 moves, curving inwards to the centre of the leaf, and gradually the other 

 marginal tentacles incurve over the insect (Fig. 42 B). The glands then secrete 

 an acid liquid containing a digestive ferment which acts upon and dissolves the 

 soft parts of the insect, and the products of this digestion are absorbed. 



Plants which do not possess chlorophyll are incapable of using 

 carbon dioxide as carbonaceous food, but require more complex 

 carbon- compounds. Such plants are, all Fungi, and among the 

 higher plants, Cuscuta (Dodder), Orobanche (Broomrape), Neottia, 

 etc., though in some of these latter, a small, but altogether insig- 

 nificant quantity of chlorophyll has been detected. These plants 

 absorb the complex carbon-compounds which they require, either 

 from living animals and plants, or from the decaying remains of 

 animals and plants : in the former case they are termed parasites, 

 in the latter saprophytes. 



The most common parasites are those which are nourished by other plants, 

 termed hosts, and each parasite has its own peculiar host, and possess peculiar 

 root-like organs, the hanstoria (see p. 66), by which they are attached to the 

 host and absorb their nutriment. They frequently have a prejudicial effect 

 upon the host, and sometimes cause malformations, such, for instance, as the 

 " witches' brooms " in the Silver Fir, which are produced by a parasitic rust- 

 fungus, the JEcidium elatinum. Less common are parasites on animals, such 

 as the Schizomycetes (Bacteria, etc.), which cause various diseases, and other 

 Fungi, like Entomophthora, which is parasitic on flies, and Cordyceps, which 

 is parasitic on caterpillars. 



Some of these parasites, Cordyceps for instance, can live saprophytically to 

 some extent ; these are potential saprophytes. On the other hand, there are 

 potential parasites, that is, plants which can live almost as well parasitically as 

 saprophytically, such as Agaricus melleus, which grows equally on living trees 

 and on dead logs. 



It is remarkable that certain plants which possess chlorophyll are nevertheless 

 parasitic in habit ; for instance, Viscum (the Mistletoe) which is parasitic on 

 various trees, Khinanthus (the Battle) and other Scrophulariaceae, also Thesium 

 (Bastard Toad-flax), which are attached to the roots of other plants by their 

 haustoria. The nutritive processes of these green parasites are not yet fully 

 understood, but it seems probable that they absorb from their hosts the sub- 

 stances which they should normally obtain from the soil, though in a somewhat 

 modified form. 



The great majority of the saprophytes are Fungi, such as the various Agarics 

 which grow in the soil of woods (humus) which is formed by decayed leaves and 

 is rich in organic matter ; the Moulds and Yeasts which grow in saccharine 



