274 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



the tubular or saclike asci springing from the base, and not 

 indifferently from the walls of the perithecium. Most of them 

 grow on living leaves, and are very destructive, either by 

 directly diverting the nutritive juices from their proper office 

 and appropriating them to their own use, or by blocking up 

 the stomates and impeding the free action of the rays of light 

 and of the surrounding atmosphere. A few only occur indif- 

 ferently on various dead substances, and these approximate a 

 higher group. 



279. In an early stage of growth they present the appear- 

 ance of moulds (Fig. 20, a), which is so much the stronger, 

 because the filaments bear reproductive bodies, and sometimes 

 do not advance further. In this condition, they are the 

 plagues of many of our cultivated plants, constituting the 

 mildew of the vine, rose, turnip, &c., appropriating the juices 

 of the plant like true parasites, and in consequence producing 

 disease, and even death. While in some species, however, there 

 is a disposition to preserve this original condition, in others 

 there is almost as strong a tendency to bear fruit. Whereas in 

 the vine mildew no ascigerous sporangia have yet been found, 

 in the hop and pea mildew, the perithecia are almost contem- 

 poraneous with the mucedinous fruit, and in consequence 

 there is at one and the same time a double mode of propagation. 

 But, in addition to the mucedinous threads and perithecia, 

 there is a third form of fruit, either resembling the perithecia 

 in outward form, or assuming an ovate outline, with a more 

 or less acuminate apex, in which naked spores of extreme 

 minuteness are produced in numbers almost incalculable. 

 These may possibly be representatives of male organs ; but, 

 allowing this to be the case, like such representatives amongst 

 many Alg£e, there is much probability that they are reproduc- 

 tive. A fourth form of fruit, similar in essential characters to 

 the third, is formed occasionally within perithecia, resembling 

 exactly those which are ascigerous ; and even a fifth occurs, 

 consisting of minute bodies in the organized protoplasm of the 

 moniliform joints of the threads, which spring perpendicularly 

 from the mycelium ; joints which under other circumstances 

 are themselves immediately reproductive. 



