82 BOTANY. [CHAP. n. 



East Indian Archipelago, the vegetative part is reduced 

 to an absorbent portion that is completely buried in 

 the substance of the host ; the flowers, or reproductive 

 portion, on the other hand, often attaining enormous 

 dimensions, the flowers of Rafflesia Arnoldi measuring 

 nearly three feet in diameter. This loss of balance due 

 to a parasitic habit between the vegetative and repro- 

 ductive parts of plants, has its counterpart in the animal 

 world, where such parasites as the tapeworm are reduced 

 to the reproductive portion, and, like the last-named 

 animal, many parasitic plants belonging to the fungi 

 spend different periods of their existence or life-cycle on 

 different host-plants ; thus the common " rust " of wheat 

 and other cereals, Puccinia graminis appears in the 

 early summer on the leaves and stems as bright rust- 

 coloured streaks, hence the popular name ; if the spores 

 or reproductive bodies which constitute the rust-co- 

 loured powder be examined under the microscope, they 

 will be found to consist individually of a single cell with 

 a bright brown rough cell-wall ; these spores as soon as 

 mature, if carried by wind or rain on to other grass-leaves 

 or stems, germinate at once, the mycelium penetrating 

 into the tissues, where in a short time it produces similar 

 rust-coloured streaks, that burst through the epidermis 

 and appear on the surface, ready to be transported and 

 form the starting-point for a new colony ; in this manner 

 the rust spreads rapidly during the summer months, 

 and being a thorough parasite, robs the host of a consi- 

 derable amount of assimilated food originally elaborated 

 for its own use ; towards the autumn, the streaks on the 

 stem and leaves become somewhat darker in colour, and 

 if a portion of the powder is examined under the micro- 



