482 MANUAL OF BOTANY 



roots serve the same purjiose as the velamen of the orchids- 

 Besides these roots, thus adapted to absorb watery vapour from 

 the air, epiphytes have frequently others which are closely 

 applied to the surface of the bark on which they are growing. 

 These are often strap-shaped, and cling very closelj^ to the tree, 

 absorbing from the bark the soluble products of its decomposition 

 and any mineral debris that may be accidentally carried thither. 

 The small amount of such food stuffs available will explain the 

 relatively large development of the root system, which is in much 

 greater proportion than in ordinary land plants. 



Parasites are another class of plants that have undergone 

 much modification of structure in consequence of their mode of 

 life. The parasitic habit is seen most completely in the group 

 of Fungi, but it is by no means confined to them. We find many 

 cases of partial or complete parasitism among flowering plants. 



The fungus which is parasitic, derives all its nourishment 

 from the plant or animal whose tissues it has invaded. Others 

 of the same group are not parasitic, but live upon decomposing 

 organic matter, being known as saprophytes. Their mode of 

 nutrition is, however, essentially the same. In no case is 

 chlorophyll present in the plant body, a fact which causes 

 it to be unable to utilise and work up the food materials 

 which green plants absorb from the air. Instead therefore 

 of absorbing their carbon in the form of CO.,, these parasites 

 must take it in the form of an organic compound of some 

 complexity, which is usually some form of sugai^. Their 

 nitrogen can be absorbed much as that of a green plant, 

 but they appear to utilise compounds of ammonia in preference 

 to nitrates. No doubt their protoplasm is ultimately fed with 

 the same materials as is that of the higher plants, but they 

 lack a great deal of the constructive power of the latter. 



Associated with the absence of the constructive processes 

 which depend upon the presence of chlorophyll we have a great 

 degradation of the plant structure. Their body is usually 

 composed chiefly of delicate hyphse, which ramify in the 

 nutrient substratum, either living or dead, and which absorb 

 elaborated products of some complexity freely by their whole 

 surface. There is therefore no need of differentiated absorbing 

 or conducting tissues, which are accordingly not developed. A 

 further consequence of the ease with which they obtain their 

 food is the readiness with which vegetative and asexual repro- 

 duction is brought about : hence sexuality in many cases is non- 

 existent among them. 



