894 
Wilson.—Plant Distribution in the 
into direct competition with each other. The rhizomes of Adoxa Moscha - 
tellina , on the other hand, are found at considerably less depth in the soil, 
while the tubers of Ranuncidus Ficaria occur still closer to the surface. 
The society, as a whole, is complementary, the uppermost soil layers being 
occupied by Ranuncidiis Ficaria, the intermediate by Adoxa Moschatellina, 
and the lowest by Endymion nutans and Arum maculatum. 
A full account of the life-history and morphology of Endymion nutans 
has been given by Woodhead (17 and 18), and this investigator has 
described the passage of the bulb downwards to the deeper layers of the 
soil during the development of the mature plant. A similar phenomenon 
has been mentioned by Oliver (14) in the case of Adoxa Moschatellina , and 
a similar process takes place in plants of Arum maculatum produced from 
seed (Scott and Sargant, 15). The soil found in copses on the Thanet 
Sand is particularly well adapted for the descent of the rootstock, since 
it consists typically of a loose sandy loam without stones of any kind. 
Little resistance is offered to the descent of the plant, and this circumstance 
probably partially explains the abundance of the species in question. 
As already mentioned many of the areas described as Thanet Sand 
contain a considerable quantity of pebbles in the soil. The presence of 
pebbles evidently has an adverse effect on bulbous and tuberous species. In 
the case of Endymion , Adoxa , and Arum the sinking of the rootstock 
is obstructed both by the stones and the increased hardness of the soil. In 
consequence these species are absent or only occur in greatly decreased 
quantity wherever the soil contains a considerable proportion of pebbles. 
Ranunculus Ficaria also disappears from such soils, but in this case 
the limiting factor is, perhaps, the lower water supply. Of the four plants, 
the Bluebell can, apparently, best withstand the increase in quantity of the 
stones, for although absent on some soils of the Woolwich and Reading 
deposits, it is generally sparingly found both on these and on the Oldhaven 
beds. 
As would be expected, these plants are absent from or only sparingly 
found on the majority of the Chalk areas, since in these situations the soils 
are generally shallow and contain numerous fragments of chalk. Endymion 
nutans, Adoxa Moschatellina , and Arum maculatum do, however, occur 
frequently wherever the soil is of greater depth. These three species are 
also found on the deeper soils of the Clay with Flints, but are absent from 
or only sparingly found on the London Clay. This fact is somewhat sur¬ 
prising as these two soils are so nearly similar both in mechanical and 
chemical composition. The soil of the London Clay is, however, much 
stiffer and denser than that of the Clay with Flints and, in consequence, 
offers more resistance to penetration by the rootstock. This circumstance 
alone, however, seems inadequate to explain the abundance of these plants 
on one deposit and their scarcity or absence on the other. 
