WEEDS— PECULIARITIES AND DISTRIBUTION 423 



the fields. The weeds dealt with only include those found 

 along with the crop. In each season's work (1910, 191 1) over 

 a hundred different species of weed plants, representing above 

 seventy genera, have been met with. Some of these species 

 occurred very frequently, being universally distributed in the 

 localities in which they were found, whilst others, perhaps 

 thirty in number, were only seen once or twice each year, being 

 very local indeed in distribution. The two seasons were 

 entirely different in character, that of 1910 being cold, wet and 

 relatively deficient in sunshine, whilst that of 191 1 was excep- 

 tionally hot and sunny, the deficiency in the rainfall causing 

 conditions of brought to prevail during extended periods. It 

 is quite possible that some of the results obtained are affected 

 by seasonal variations and not only by soil conditions. 



It is already evident that certain plants are quite universally 

 distributed, occurring on any and every type of soil, whether 

 chalk, clay or sand ; others on the contrary are far more 

 localised and entirely or chiefly confined to certain definite 

 types of soil, so much so that particular species gradually come 

 to be recognised as symptomatic of certain soils. 



A. Weeds of Clay Soils and Heavy Loams. — The soils 

 described as clay are very fine in texture, being composed 

 essentially of a large proportion of tiny particles of silt bound 

 together by a small proportion of true clay, silicate of alumina, 

 which is most adhesive in nature. The finely grained texture 

 renders the force of capillarity very great, as the interspaces 

 between the particles are so small ; as a result such soils are 

 most retentive of water, holding a large percentage under 

 ordinary conditions. This tends to make the clays somewhat 

 heavy and cold ; there is also a danger of water-logging, as 

 superfluous water drains away with difficulty. Water-logged 

 soils are ipeculiarly harmful to plant life, on account of the 

 deficiency of air in their interspaces. Unless the under- 

 drainage of clay soils is very efficient, so that the effect of these 

 disadvantageous conditions is minimised, the number of species 

 of plants found upon them is somewhat restricted, as all plants 

 which are at all sensitive to an excess of soil moisture are 

 lacking, leaving the field clear for those which are less exacting 

 in their demands. The flora on the clays, however, is not very 

 sharply marked out, as there are so many gradations from heavy 

 clay down to heavy loams and thence to lighter loams, 



