45] CHAP. II. PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 189 



Besides these we find in the ash of many plants though they 

 cannot be regarded as essential to nutrition the following ele- 

 ments : sodium, lithium, manganese, silicon, iodine, bromine, and 

 in rare cases, also aluminium, copper, zinc, cobalt, nickel, stron- 

 tium, and barium. Fluorine must also exist in vegetables, for it 

 is found in a perceptible quantity in the dentine of animals which 

 feed directly or indirectly on vegetables. 



The essential elements of the food will now be severally dis- 

 cussed. 



Carbon. Plants which possess chlorophyll obtain their carbon 

 mainly from the air (or, in the case of submerged plants, from the 

 water) in the form of carbon dioxide. The absorption of carbon 

 dioxide is, however, limited to those cells which actuall}' contain 

 chlorophyll, and it can only go on even in those cells so long as they 

 are exposed to sufficiently intense light. 



Although plants possessing chlorophyll can and do use cai'bon dioxide 

 as carbonaceous food, yet there is reason to believe that they may supple- 

 ment this by absorbing more complex carbon-compounds. In certain cases 

 (e.g. Drosera, Dionsea, Utricularia, etc.), green plants are provided with a 

 special mechanism, in the form of modified leaves, for obtaining a supply 

 of organic carbon-compounds. Such plants are said to be insectivorous. 

 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. 33, p. 48). 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 stimu- 

 lates the tentacle, and it moves, curving inwards to the centre of the leaf, 

 and gradually the other marginal tentacles incurve over the insect (Fig. 

 33 B). The glands then secrete an acid liquid containing a digestive 

 enzyme 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 organic substances formed 

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

 in the latter saprophytes. In some cases plants destitute of chloro- 

 phyll obtain their carbonaceous food from green plants, without^ 



