§ X.] Parasitism. 113 



the attacks of the parasite, and unequal power of resisting them 

 according to the relative amount of water which they contain. 

 Since in these cases youngerplants have more water than theolder, 

 a predisposition dependent on age is also indicated accordingly. 



In cases where the parasite causes a disturbance, which we call 

 sickness, of what by experience is regarded as the normal vege- 

 tation of the host, if the predisposition is individual we speak 

 usually of a sickly predisposition. This may be correct, in 

 so far as the predisposition to the attack of the parasite may 

 be connected with deviations from the state which is from ex* 

 perience termed the sound state. But it need not always be 

 correct, for there is no reason at all why the disposition for the 

 attack of the parasite should in every case indicate a condition, 

 which must be called sickly even when there is no parasite 

 present. The above-mentioned example of the predisposition 

 varying with the age is a suflScient proof of this. Here we must 

 distinguish between one case and another, and care is required 

 in determining each individual case. 



An example may help to make this still more plain ; it is that 

 of a case which is comparatively very accurately known. The 

 common garden-cress, Lepidium sativum, is often attacked by 

 a parasitic Fungus of comparatively large size, Cystopus can- 

 didus. In consequence of this it shows considerable degene- 

 ration, swellings, curvatures of the stem, and often also of the 

 fruits, and on these parts and on the leaves white spots and 

 pustules subsequently turning to dust, which are formed by the 

 sporogenous organs of the Cystopus, and give the entire phe- 

 nomenon the name of the white rust in cress. This is a case of 

 disease, and so striking that every one notices it at once with the 

 naked eye. Now we find in a bed of cress at about flowering 

 time a certain number of rusty plants, two for example or 

 twenty. They are in the middle of the other hundred or 

 thousand plants, and these are healthy and free from the Fungus 

 and continue so till the period of vegetation is at an end. This 

 is the case, though the Cystopus forms countless spores in the 



