WEEDS AND WILD PLANTS. 341 



grow where they are^ not wanted, and which interfere more or less 

 seriously with horticulture. We are very much troubled with a lovely 

 plant called the Marchantia (see Liverworts), which springs up in my 

 artificial bogs and sandstone rocks, thereby destroying my bog plants, 

 which are difficult enough to grow without the interference of the 

 Marchantia. It has a sort of green leaf, which sends up a forest of 

 stalks, from which the spores emanate. Although so troublesome a 



FIG. 783. Hydrocotyle vulgare. FIG. 784. Anacharis. 



weed, it is welcome in small quantities, and is one of our finest natural 

 plants. 



We have as troublesome a plant, which lives in water, in the 

 Anacharis (fig. 784), or, as Sowerby calls it, the Elodia canadensis, as 

 we have in the Marchantia^ which lives on land. It is naturally a North 

 American plant, and was first seen in this country in 1842, but it has 

 now spread all over Great Britain and Ireland. It does not grow in 

 very deep water, and prefers water with manure. Mr. Thornthwaite 

 tried the experiment in one of my greenhouses of placing a weighed 

 portion in distilled water, in the river water which then received the 

 sewage of Croydon, and in a mixture of the two. In a short time 

 the growth of the plant in river water far exceeded that of the plant 

 placed in pure water, and that in the mixed waters had an intermediate 

 growth. This shows that an important effect of preventing the pollution 

 of rivers would be to lessen the quantity of this most troublesome 

 plant. It is a trial of patience for a fisherman when his hooked 

 pike gets into this weed. I was informed by Professor Owen that 

 swans eagerly devoured the Anacharis ; accordingly, acting upon his 



