Pethybridgk & Laffkrtv — Disease of Tomato and other Plants. 491 



the average they measure 40 fi by 27ju, and they are, therefore, somewhat 

 larger than those of P. erythroseptica. 



The branches of the sporangiophores do not differ markedly from the 

 ordinary mycelial hyphae, and are as a rule long and straggling. Careful 

 observation has shown that the first formed sporangium is terminal, and that 

 the branch of the sporangiophore which bears the second arises near the base 

 of the first sporangium, which thus becomes pushed aside in the manner first 

 described by de Bary for P. infestans, but no local swelling of the 

 sporangiophore occurs, as in that species. More than three sporangia attached 

 at one time to a branched sporangiophore have never been seen. (See fig. 4, 

 PI. XL VII.) 



The mode of germination of the sporangia has been carefully followed, 

 and it resembles very closely that occurring in P. erythroseptica (5). The 

 contents of the sporangium .become more coarsely granular, contraction of the 

 contents from the sporangium wall occurs to some extent, and aggregation 

 into a number of zoospore units takes place (fig. 3, PL XLVIL). Suddenly 

 the thicker, hyaline, apical portion of the sporangium wall becomes stretched 

 or expanded into an' almost spherical, extremely thin-walled sac, into which 

 the whole or the major portion of the contents of the sporangium immediately 

 passes. Practically at the same moment the wall of this sac dissolves, and 

 the contents of the sporangium break up into a number of zoospores which 

 swim off in all directions. Any contents which may not have escaped from 

 the sporangium become resolved at the same time into zoospores which may 

 swim out of the now ruptured sporangium, or may remain within it and 

 germinate there. 



The zoospores are lemon-shaped with a distinct longitudinal groove on 

 one side, from the base of which two cilia arise, one directed forwards and 

 the other backwards. After a short time each zoospore comes to rest, loses 

 its cilia, assumes a more or less spherical shape, secretes a thin wall and 

 produces a germ-tube. When at rest the zoospores measure from 10^ to 

 15/x in diameter, and they are, therefore, somewhat larger than those of 

 P. erythroseptica. 



When a sporangium has liberated its zoospores it not infrequently 

 happens that the branch of t.he sporangiophore which bore it grows directly 

 onwards and forms a second sporangium. If this branch becomes elongated 

 the second sporangium is formed outside of and beyond the original one, now 

 empty, as is shown in fig. 7, PI. XLVII. But if this branch remains short, 

 the second sporangium, being almost sessile, remains within the first formed 

 one ; and a third may be developed within the second as illustrated in fig. 8, 

 PI. XLVIL A similar state of things has been described by Kobinson (9) for 



