My dear Sir

I had the pleasure of forwarding, yesterday, (per S.E.R.) plants of Nitella opaca which I hope have reached you safely. A little gravel covered with sand is what I have used for planting Characeæ & rain or boiled water.¹ The struggle for existence in the gravel pit pools at Mitcham (whence I obtained these spns) has much interested me.² I have noticed comparatively new pools with only Confervæ³ & perhaps a little grass, next year I have found a single patch of the Nitella, the following year full of the Nitella with a little Callitriche Zannichellia &c the next year little or no Nitella more Callitriche, more water grasses and sometimes Myriophyllum, Elodea &c but I don’t think in any instance have I found the Nitella in the same pool for more than 3 years during the last six seasons.⁴ It seems to point to Nitella opaca being one of the first plants capable of existing in new gravel pits, but not able to hold its own when the conditions become suitable to other plants.

The limited duration of some of the Nitelleæ, more especially the Tolypella, in the same pool, is very remarkable. To give an instance— In a park near Kelvedon there are several small old established ponds which M^r. Varenne of that place, a very accurate observer, has known for 30 or 40 years— some 20 years ago he found in one of these pools Tolypella intricata, another year it occurred in a neighbouring pond and about 4 years ago, a pool some $\frac{1}{4}$ of a mile distant was quite full of it, then it again disappeared, having only been found for one year in each pond⁵

Much regretting to hear you are so unwell,⁶ | Believe me | Very truly yours | Henry Groves

Cha^s. Darwin Esq LL.D., F.R.S.