DEVELOPMENT OF PLANTS 463 



Evening Primrose family, witii one of which, Oenothera (Fig. 323, 

 A, B), deVries worked out his theory of mutation, and the 

 common ditch and pond aquatics — the water milfoil {Myrio- 

 phyllum) and mermaid weed (Proserpinaca) . 



The purple loosestrife (Lyihrum) illustrates the characters of 

 the lower members of the order before epigyny has become estab- 

 lished (Fig. 322). The wand-lilce branches of this introduced 

 plant with their terminal spikes of purple flowers are becoming 

 rather common around the borders of marshes and water ways. 

 The cup formed by the calyx and receptacle bears on its rim 

 alternately with calyx lobes the rather twisted petals and at its 

 base eight to twelve stamens. The pistil is composed of two 

 carpels. This flower has become historic because of the atten- 

 tion that has been given to its devices for crossing. Fig. 322 

 shows that the sporophylls are of three lengths, short, medium 

 and long, an arrangement called heterostyly. It can readily be 

 seen that the three different lengths of the styles correspond 

 exactly with the position of the anthers, consequently whatever 

 part of the proboscis or body of the bee or butterfly in visiting 

 these flowers comes in contact with one of the sets of open 

 anthers, exactly the same region of their bodies will touch the 

 stigmas as soon as they visit another flower with a corresponding 

 length of style. The spores from the three lengths of stamens 

 differ in size and color, and it has been demonstrated that better 

 results follow when crossing is effected between sporophylls of 

 the same length, i. e., long with long, short with short, etc. 

 Heterostyly arose in many of the orders, as among some of the 

 knotweeds, buttercups and rose families and in the gentians, 

 primroses, forget-me-not, etc. 



A higher and more characteristic type of the order is seen in 

 the Oenothera and in the great willow-herb {Chamaenerion, Fig. 

 323). This latter plant flourishes in rather dry soils, forming 

 large colonies by its underground stems and gaining in con- 

 spicuousness through its terminal racemes of large deep purple 

 flowers. The parts of the flower are in fours. The receptacle 

 forms the ovary and the mass growth of perianth and stamens 

 results in the development of a long tube which forms at its 



