370 DICOTYLEDONS AND CONIFERS 



It is frequently found to be very much infested with the common 

 cabbage blight Aphis brassicce. 



In A. R. Wallace's Darwinism he mentions the fact, communicated 

 to him by Mr John Enys, that: 



a natural remedy to the water-cress has been found by planting willows on 

 the banks. The roots of these trees penetrate the bed of the stream in every 

 direction, and the water-cress, unable to obtain the requisite amount of 

 nourishment, gradually disappears. 



This is no doubt quite true for narrow streams, with a good flow 

 of water. In such situations also Elodea Canadensis tends to displace 

 it; and I have noticed in some parts of the Avon at Christchurch, 

 and in tributary streams, that a species of Nitella can strangle both 

 of them. But watching shallow ponds near Dunedin, I have noticed 

 that unless kept severely in check, the water-cress can put Elodea, 

 Aponogeton and species of Nymphcea right out of competition in a 

 year or two. 



A few years after their introduction into the streams in the Canter- 

 bury district, water-cress plants grew to gigantic proportions, being 

 as much as 14 feet in length, and stout in proportion. The size is 

 now quite normal. 



In Europe the flowers are fertilised by the honey-bee (Apis mel- 

 lifica). 



Barbarea prcecox, R. Br. American Cress 



First recorded in Kirk's list of introduced plants on the Great 

 Barrier Island in 1867; and in Wellington in 1877. In the Manual 

 (1906) Cheeseman reports it as not uncommon in waste places and 

 roadsides in both islands. (Fl., Oct. to Dec.) 



Barbarea vulgaris, R. Br. Winter Cress 



Said to be common on Otago Peninsula; I am not sure of the 

 identification. 



Arabis hirsuta, Scop. Rock-cress 



Reported as occurring near Ashburton by W. W. Smith. 



Alyssum calycinum. Linn. 



First recorded by Armstrong in 1879 as occurring in Canterbury. 

 Cheeseman reports it from both islands as not uncommon on road- 

 sides and in waste places. Petrie states that he has found it in Central 

 Otago, growing up to 3000 ft. (Fl., Nov. to Jan.) 



Alyssum maritimum, Linn. 



First recorded in Hooker's list in 1864. Armstrong in 1871 re- 

 corded it among plants from Canterbury. Kirk in 1877 reports it 

 from two points near Wellington. In 1906 Cheeseman states that it 



