GREAT YELLOW WATER CRESS 83 



and being taller it makes a greater show above water than Water 

 Cress. It has almost entire broadly lance-shaped, sometimes coarsely- 

 toothed, dark-green foliage, and being deeply rooted by means of long 

 stringy roots, like a Water Dropwort, it spreads out from the banks for 

 some distance in deep water, forming a fringe along a canal bank, or in 

 shallow streams filling the channel entirely. 



It may be recognized by its Nasturtium habit, combined with the 

 yellow flowers, the petals twice as long as the calyx, the flower-stalks 

 spreading or turned down ; the pouch is egg-shaped, with a stigma with 

 a pin-head, and the seeds are small, the silicules being ellipsoid and 

 swollen, the pods being shorter than the flower-stalks, and there is no 

 vein on the pouch. 



This plant grows to a length of 4 ft. It is in flower from June 

 to September. It is a herbaceous perennial, and reproduced by seeds 

 or by division. The structure of the flower is similar to that of N. 

 sylvestre, in which at the base of the flower between every 2 stamens 

 there is a green, fleshy honey-gland. In this there are 6 nectaries in 

 a ring at the base. The anthers of the 4 longer stamens are nearly 

 on a level with the stigma, the other 2 are deeper, and all are turned 

 towards the centre. The anthers spread out when the flower is open, 

 and open towards the stigma. Or they may make a half-turn and so 

 avoid possible self-pollination. Visitors insert the head between the 

 stigma and stamens, and each side of the head is dusted; while the 

 insect remains in the same flower the same side touches the stigma, 

 but if it visits others the opposite side may touch the stigma, and 

 cross-pollination will follow, while if the same insect inserts its head 

 into the same flower several times it may cause self-pollination. In 

 wet weather the anthers of the long stamens touch the stigma and 

 the plant is self- pollinated. It is visited by Hymenoptera (Tenthre- 

 dinidae, Tenthredo], Diptera (Empidse, Empis, Syrphidse, Rhingia, 

 Syritta, Eristalis], 



Great Yellow Water Cress is dispersed by the plant's own agency. 

 The seeds are small, and are dispersed after the tension of the oblong 

 pods, when dry, has caused the pod to open lengthwise, and scatter the 

 seeds to a distance. 



It is a hydrophyte and aquatic, rooted in the alluvium of the river- 

 or lake-bed in the reed swamp. 



There are apparently no fungal or insect pests that infest 

 this plant. The second name amphibia (Latin) alludes to its 

 amphibious habit. This plant is called Bellragges, Water Charlock, 

 Laver. 



