NUTRITION OF SEMI-PARASITES 225 



becomes possible ; without them the cell-sap of the storage-tissues 

 would soon become so concentrated that a further movement of 

 material into the cell by osmosis could not occur. Moreover, the 

 change of a soluble osmotic substance into an insoluble form 

 prevents the turgidity of the cells from becoming excessive. 



Ex. 133. — Cut transverse sections of last season's branches of ash and other 

 trees in winter : place them for a moment in iodine solution (see Ex. 85) and 

 then mount in water. Examine with a low power and note in what tissues 

 the starch is most abundant. 



5. Nutrition of semi-parasites and semi-saprophytes. — Cer- 

 tain green plants, in addition to their power of forming organic 

 compounds from carbon dioxide, water, nitrates and other 

 simple inorganic substances, appear to derive some organic 

 materials ready formed either from other living plants or from 

 humus. 



To the former class belong Yellow-rattle (Rhinanthus Crista- 

 galli L.), Eyebright {Euphrasia officinalis L.), Red-rattle {Pedicu- 

 laris sylvatica L.), species of Melampyriim, and other semi-para- 

 sites not uncommon in meadows and pastures. Certain portions 

 of the roots of these plants attach themselves by small haustoria 

 to the roots of other plants growing near them and no doubt 

 absorb a certain amount of organic substance from the latter, 

 for unless they become attached in this manner to other plants 

 they do not grow satisfactorily. 



Many flowering plants, such as bird's-nest orchis {JVeottia) 

 and species of Monotropa, possess few or no chloroplasts, 

 and live upon humus : numbers of plants, such as Heaths, 

 Rhododendrons, Azaleas and Winter-green {Pyrold) belonging 

 to the Ericaceae, Beech, Hornbeam and other representatives of 

 the Cupuliferse, as well as pines and Coniferse generally, while 

 possessing chloroplasts appear to supplement their own manu- 

 factured supply of organic material by absorbing organic com- 

 pounds from the decaying humus or leaf-mould in which many 

 of their roots are found growing. 



p 



