iv IRVINE AND WOODHEAD'S EXPERIMENTS 203 



Woodhead have shown. They have seen that crabs 

 living in sea-water from which chloride of calcium is 

 excluded, cannot, after shedding their exoskeleton, 

 form a new one even when sulphate of lime and chloride 

 of sodium are present in the water : chloride of calcium 

 is quite necessary for the formation of the crab shell. 

 Considering now external form, we see that it may 

 be influenced by external influences, as well as internal 

 constitution or physiology. Here are some instances 

 out of many. M. Languet de Sivry, some fifty years 

 ago, 1 noticed that seeds of short-rooted carrot, when 

 sown in a particular soil, in the alluvial deposits 

 formed by a small river in France, yielded imme- 

 diately, during the first generation, a number of long- 

 rooted plants, either white or yellow, whose roots 

 were very much larger than those in the parent plants. 

 The seeds of the best, or less deformed plants, were 

 selected, and sown in the same soil. The result was 

 that in the second generation hardly any roots were 

 found of the short type, and most were exactly similar 

 to the common wild form. Again, Petermann (Con- 

 tribution a la Chimie eta la Physiologie de la Better ave 

 a sucre, Brussels, 1889) notices the fact, familiar to all 

 horticulturists, that when a plant beet for instance 

 is grown in poor soil, its root becomes much longer 



1 Cf. Societe Roy ale et Centrale d' Agriculture, 2nd series, vol. ii., 

 1846-7, p. 539. 



