SECTIONAL TRANSACTIONS.— K. 425 



It is believed, therefore, that in the life-history of this fungus there is one 

 nuclear fusion, namely that in the penultimate cell of the crozier, and the 

 compensating reduction of the chromosome number occurs in the first of 

 the three divisions in the ascus. 



Mr. C. G. Chesters, Mr. H. E. Croxall and Miss K. M. Keene.— On 

 certain fungi having a Libertella imperfect stage (3.15). 



After a brief review of the characteristics of the Allantosphaeriaceae 

 von Hohnel, illustrated by references to species of the genera commonly 

 occurring in Britain, certain species of Diatrype, Eutypa, Diatrypella and 

 Anthostoma are described in greater detail. Special attention is paid to 

 the structure of the stromata and the development of the imperfect stage 

 both on typical host substrata and on agar media. It is shown that a 

 Libertella stage has been developed in single ascospore cultures of the 

 majority of the species studied and that similar Libertella spores have been 

 collected in association with the ascophorous stage on naturally infected 

 hosts in certain instances. Some species have also a Maetnospora stage 

 occurring on the same mycelium as the Libertella ; this is considered to 

 represent a microconidial condition. The importance of the similarity of 

 the imperfect stage in the several genera of the Allantosphaeriaceae is 

 emphasised and its bearing upon the relationship of these genera with 

 certain other Pyrenomycetes is discussed, 



Mr. A. E. Vines and Dr. A. H. Campbell. — A physiological effect of 

 Lophodermium macrosporum on the needles of Picea excelsa (3.35). 



In a Spruce the needles usually fall rapidly should the shoot bearing them 

 be damaged or cut from the tree. Abscission takes place at the junction of 

 needle and peg, leaving the bristle-like peg attached to the shoot. When 

 Spruce needles are infected by Lophodermium macrosporum, it is noticeable 

 that they cannot be easily detached even after the shoot has been cut from 

 the tree for several months. If such needles are pulled from the shoot the 

 peg will invariably be detached as well. It is evident that the fungus 

 prevents the proper functioning of the abscission mechanism situated between 

 the needle and the peg. It can be shown that abscission is brought about 

 by water loss from the needle causing a hygroscopic contraction of the base. 

 This contraction is greater than the contraction of the peg tissues, with the 

 result that stresses are set up which ultimately bring about abscission. 

 To operate this mechanism it is essential to have a considerable and rapid 

 water loss from the needle. Infected needles are provided with a black 

 ring or zone of fungal cells at the base and with mycelial aggregations which 

 block the stomata. It can be shown experimentally that the fungal plates 

 together with the sub-stomatal sclerotia effectively diminish water loss and 

 hence prevent the abscission mechanism from functioning although the 

 needle may have been dead for some time. 



Dr. N. T. GiLh.— Observations on the viability and dormancy of weed seeds 

 (3-55)- 



Our knowledge of the stage of development at which seeds of various 

 plants become capable of producing a new plant is not very considerable. 

 This knowledge is of some practical importance ; therefore observations 

 have been made on the germination of seeds collected from plants cut down 

 at various stages of growth. 



In some cases plants cut down in the flowering condition ripen seed 







