The Scott is J i Naturalist. 



117 



infested in this manner had previously been exposed to direct 

 sunlight. This method is peculiarly successful in enticing the 

 Phytoptus from the covered parts of the buds. I employed this 

 process to make closer investigations into the length of stay of the 

 parasites on the plants without injuring the latter. 



Interesting phenomena occurred if the plants were affected only 

 to a slight extent, or were but slightly susceptible, i.e. where 

 though the infection was made to a considerable amount and at 

 the proper season no very striking alterations in form followed. 

 In this context the word susceptibility (Empfindlichkeit) may be 

 employed. The infected plants produced abnormally-formed 

 leaves, and single or a few or many double flowers, with various 

 degrees of doubling (petalody of stamens and carpels, super- 

 numerary doubled organs, or proliferous flowers), according to 

 whether few or many individuals of Phytoptus were transferred to 

 them. Apart from such abnormal leaves or flowers the plants 

 showed quite a normal appearance ; they looked healthy and not 

 in any way in a condition to lead one to pre-suppose the presence 

 of parasitic animals ; as, indeed, similar examples have been ob- 

 served by various botanists, without their having a conception of 

 the cause of the doubling of the flowers. 



Among the very susceptible food-plants are many (but not all) 

 species of Valeriana, especially V. tripteris, dioica, officinalis, and 

 supina, two species of Centranthus, viz. C. Calcitrapa, and macro- 

 siphon, and Fedia Cornucopia?. In these plants both leaves and 

 flowers assumed an abnormal aspect. 



The time at which the consequences of infection first became 

 noticeable was earlier or later in the different species. In those 

 species whose leaves suffered severely, I could frequently detect 

 the first abnormal leaves in twelve or fourteen days after infection. 

 But at this time they were not fully developed. The margins 

 were inrolled along shorter or longer spaces ; the rolled portions 

 became broader after some weeks, and the wider parts were then 

 beset with the characteristic teeth. The rolls consist of undiffer- 

 entiated cellular tissue, which does not pass into the permanent 

 condition till much later than the tissue of the other parts of the 

 leaves. The cellular tissue of the excrescences on the upper 

 surface of the leaves is also late in passing into the permanent 

 condition. On the succeeding leaves the phenomena are repeated 

 with equal, greater, or less intensity, one or more leaves often 



