VITALITY OF WEED SEEDS 



81 



The number of species found at different depths is also 

 very instructive. As might be expected, in present day arable 

 land and land that has been grassed over for comparatively 

 short periods most of the species are represented in the top 

 six or seven inches, a less number occurring at lower depths. 

 When the land has been under grass for longer periods, how- 

 ever, many of the arable species tend to disappear from the 

 upper few inches, as most of the seeds have either been carried 

 lower or have started into growth and perished. An apparent 

 exception to this is seen in the Laboratory House Meadow, 

 but in this case, as the meadow is close to the house, doubt is 

 thrown on the age of some of the buried seeds in the upper 

 inches, owing to the proximity of allotments. 



Distribution of Species of Buried Weed Seeds. 



The number of species of weed seeds that are obtained by 

 this method after burial for ten years and more is somewhat 

 limited. After the shorter periods large numbers of certain 

 species are found, especially orache (Atriplex patula), poppy, 

 knotgrass (Polygonum aviculare], field speedwell (Veronica 

 tournefortii and V. agrestis). As time goes on the poppies 

 and speedwell drop out almost entirely, but orache and knot- 

 grass are still plentiful even after many years of burial. 



In view of the fact that large crops of charlock are expected 

 and often obtained when grassed-over arable land is ploughed 

 up it is interesting to notice the evidence of the survival of 

 this weed in this experiment. 



