110 Journal of Agriculture, Victoria. [11 Feb.^ 1918. 



It spreads iu two ways: — 



(1) Yegetatively, by lateral stcloiis rooting at their ends. These 



are formed from early spring to late autumn, hut the 

 plant rarely extends in this way more than 1 to 3 feet 

 per year. 



(2) By saeds, M'hich float for a time and then sink. They ripen 



in summer — -usually from January to February — and the 

 plants flower from about the end of IS^ovember to end of 

 December. The plants at Toorourrong were just com- 

 mencing to flower on 27th November last. Ripe seeds are 

 developed only when both male and female plants are 

 present; but, even then, it often happens that no fertile 

 seed are formed for one or two years in succession. 

 Specimens of both male and female plants were found 

 in flower on the lake. 

 The seeds germinate in late summer, when, under natural conditions, 

 the water-level is low, and the growth keeps pace with the rise of the 

 water-level later on. If the water level be high the seeds may remain 

 dormant until the following summer. At a depth of 6 feet, the seeds are 

 unable to form seedlings, but if started at a depth of 2 or 3 feet, they \vi" 

 grow with the rise of the water up to 6 feet. The sudden increase of tiie 

 weed in the lake at Toorourrong was due to the level being low at a time 

 when the plant had had a successful seeding season, and the seeds were 

 able to germinate. 



Modes of TREAT:\rFNT. 



1. The most effective method would be to drain the lake, and scoop or 

 cut away not less than 3 inches of the top layer of the silt wherever the 

 plant was growing. In some parts, this would be a difficult task, for 

 where there was any depth of silt, it would take months to become firm. 



2. The mere exposure of the infected area during a hot, dry summer 

 until the plants and mud were dry would reduce the growth very greatly 

 in the following season, provided the area was allowed to dry for not 

 less than three months, and that draining took place before any seeds 

 had been foiTned. 



3. Raking the plants off the bottom from a boat is comparatively 

 ineffective, as sufficient plants and rooted stolons are left behind to 

 re-establish the weed in the following year, 



4. Cutting the weeds by dragging a knife behind a boat, or by using 

 a cutting machine — as is done on Lake Wendouree — will keep them down 

 for one season, but,, to exhaust the plants, needs repeating at least three 

 times a year. 



5. The most practicable mode of treatment would be to deepen the 

 lake by raising the banks, and then to cut the weeds as closely as possible 

 before raising the water level to the new height, and before seeding takes 

 place — that is previous to the month of January. During the dangerous 

 seeding months — January to March, or, occasionally in late seasons, 

 April — the lake should, if practicable, be kept at its full depth. When 

 once Vallisneria spiralis has got a hold, it will abnost inevitably travel 

 up to newly-submerged shallower parts, and also to areas which are 

 silting up; but it spreads slowly upAvards as compared with its rate of 

 spread with the current, however slow the latter is. Hence, by the 

 last-mentioned method of treatment, it would be possible in the future 

 to keep the weed within bounds with a minimum of trouble. 



