CHAPTER VII. — PHENOMENA OF VEGETATION. — PARASITES. 389 



Phacidium &c, must be added to the list, — species which commence their vegetation 

 on the living leaf, and complete their development by forming their sporocarps on 

 it when fallen and decayed in the next period of vegetation, and here too at the 

 expense of the reserve of food obtained from the living host. How far the building- 

 material required for the last section of the life of the plant is partly taken from the 

 dead decaying leaf has never been thoroughly investigated and can scarcely be 

 determined with entire certainty. But this supply of material can be at most only a 

 small addition, since on the one hand the Fungus-body, separated from the sur- 

 rounding substance of the fallen leaf, also reaches its normal and complete develop- 

 ment, and on the other hand it can be seen directly that the reserve material stored 

 up in the Fungus-body is used up in the course of this development. It is possible 

 that in this case also intermediate forms and shades of difference may occur, in which 

 a final stage of enforced saprophytism succeeds the parasitic vegetation, in the way 

 described above on page 373 in the case of Cordyceps. 



Section CXI. The purely local circumstances connected with the spread of 

 parasites on plants from the place of attack need no further discussion in the case of 

 hosts formed of one or few cells, because the parasite must necessarily remain 

 narrowly localised in and on so small a body ; and the remark equally applies to 

 those which like the Synchytrieae live in single cells of larger plants. 



Parasites which form mycelia in more highly organised and especially in 

 phanerogamous plants behave very differently according to the species or to the 

 segment of their development ; in the one extreme they are confined to the immediate 

 neighbourhood of the point of attack, in the other they spread widely or unlimitedly from 

 that point over or through the host. 



From among the many species which have been noticed from time to time in 

 former sections of this book and in addition to them, we may name here as examples 

 of the first category the parasites which form narrowly circumscribed spots on the 

 leaves of Phanerogams, such as many Uredineae, and among them Puccinia graminis, 

 P. Rubigo vera, Uromyces Phaseolorum, Peronospora viticola and P. nivea (Umbelli- 

 ferarum), Protomyces macrosporus, Entyloma Calendulae, species of Polystigma and 

 Rhytisma. Each distinct spot inhabited by the Fungus is the result of the growth of 

 one or occasionally of several spores. Fresh spots are added one after another on a 

 surface in proportion as new spores from any quarter, for instance from those first 

 established on the leaf, germinate there and assail it. The species of Claviceps are 

 confined to the flowers of Gramineae and Cyperaceae during the whole of the 

 parasitic portion of their life, and destroy the young ovary in the manner described 

 above. The germ-tube of one spore at least is required to infect each ovary, and it 

 attacks the ovary directly. 



Among the Fungi of the second category which spread far from the point of 

 attack are the often mentioned Sclerotinieae, Pythieae and Phytophthora, which 

 assail the host at any point and grow through it to an unlimited extent in every 

 direction, provided the external conditions are favourable. Cyslopus candidus is 

 instructive as an instance of strictly obligate parasitism. Its germ-tubes find their 

 way into every stoma in Lepidium sativum and Capsellaon which germinating spores 

 fall (page 363). But those germs only develope further which have penetrated into 

 the cotyledons, and the mycelium may spread from them through the entire host a§ 



