440 MANUAL OF BOTANY 



plants which do not enter into the composition of the actual 

 nutritive substances. The part that many of them play is 

 obscure ; indeed some of them, as sodium, seem to be quite 

 useless. Some play a secondary part in protecting the plant 

 tissues in various ways from destructive influences, some 

 enter possibly only as the medium by which the combined 

 nitro^'en, sulphur, and phosphorus are absorbed. Others, 

 especially potassium, calcium, magnesium, and iron, have a part 

 to play in enabling the more important elements to be worked 

 up into nutritive material. 



In the cases of some green plants these processes of absorption 

 are supplemented by others. In several Natural Orders there 

 are species which are semi-parasitic in their mode of life. 

 Though they possess leaves, stems, and roots, they do not live 

 entirely at their own expense, but certain of their roots pene- 

 trate the tissues of the roots of other plants near wdiich they 

 grow. The degree of this root-jjarasitism varies a good deal 

 in the different groups. These plants supplement the processes 

 we have just described by drawing elaborated food material 

 from the host which they have attacked. 



The pitcher plants, Nepenthes, Sarracenia, &c., and the fly- 

 catching plants, Drosera, Dioncea, and otliers, can also absorb 

 nitrogenous material by the special mechanisms which they 

 possess. In the pitchers of Nepenthes, &c., there is an accumu- 

 lation of water in which insects are from time to time drowned. 

 Their bodies decay or are digested, and the products of the 

 decomposition are absorbed by the tissue of the pitchers. The 

 flies and other insects captured by Drosera and its allies are 

 similarl}^ disposed of. 



CHAPTEE VII. 



THE CHLOROPLASTIDS AND THEIR FUNCTION. 



We have seen that the different materials out of which the food 

 of the plant is constructed are carried by various means to the 

 cells of the parenchyma of the mesophyll in which chloroplastids 

 are present. The chloroplastid is a small differentiated body 

 consisting of protoplasm in the meshes of whose substance a 

 green colouring matter, chlorophyll, is contained in some form 

 of solution. Alcohol, chloroform, ether, benzol, and other liquids 

 can dissolve the chlorophyll and leave the plastid colourless. 



