The E redness of Plants. 103 



An interesting case is that of opposite pairs of leaves, which 

 may be imitated with two large horizontal bar magnets placed very- 

 close to the top of a small vertical cylindrical magnet, as shown 

 in the demonstration experiment on the table. In this case the 

 mutual effect of the two equal large fields and the smaller field is 

 the production of two equal small neutral areas at right angles to 

 the bar magnets. These areas in the plant Avould become prim- 

 ordial knobs at the stem apex, and a pair of leaves at right angles 

 to the first pair would result. This is a very common case in the 

 Dicotyledons, but very rare amongst the Monocotyledons, and 

 the first pair of leaves may be taken as the seed-leaves or 

 cotyledons. 



Some Practical Applications. 



The great outstanding practical application of these purely 

 scientific facts and theories occurs in the " earthing-up " of 

 potatoes. The potato tuber is a stem structure, a swelling at 

 the end of a lateral branch which is induced to grow downwards 

 in the soil by covering the main stem with earth above the level 

 of the buds in the axils of the lower leaves. When the stem is 

 covered with earth the stomata cannot act efficiently as passages 

 for the respiratory carbon dioxide, and that gas is not used up in 

 photosynthesis because of the absence of light. The farmer and 

 the gardener, therefore, have been carrying on this particular 

 experimental reversal of geotropic curvature in the stem every 

 year in the "earthing-up"' of their potatoes. 



A minority opinion in the past has been expressed against 

 " earthing-up " as a method of increasing the production of 

 tubers, but with the new scientific basis for this process there can 

 be no doubt that, within certain limits, more earthing-up means 

 more potatoes. Enough green foliage must be left above ground 

 to produce the starch for filling the tubers ; but in Forfarshire, 

 at any rate, those farmers who earth-up their potatoes, not only 

 once but twice or more, get the heavier crops.- A comparatively 

 small amount of foliage seems to be sufficient to produce all the 

 starch that is required. 



