From Francis Darwin   24 and 25 July 1878

Botanisches Institut | Würzburg

July 24/78

My dear father,

I have tried watering Porliera out of doors, I gave four small cans full in the day & next morning it was wide open though for several days before it had been shut. The pot-plant is very unhealthy I am afraid as its leaves are dropping off at the stalk.1

I had a little talk to Sachs about heliotropism & he thinks it is no good to moulds to be heliotropic, he says that moulds which you find inside wallnuts or apples are highly heliotropic. He does not remember ever having seen a mould heliotropic in a state of nature but only when you bring them into unnatural conditions: he says potato underground stems are sensitive to light so that they won’t grow or hardly at all in light, but he doesn’t know whether they are heliotropic or not.2 I don’t quite know what part he means I will talk some more. He says he agrees with Nägeli to a large extent in thinking that certain facts cannot be explained by saying that they are useful or not useful, but he thinks they exist as properties of plants like the power of assuming a certain form is of a crystal.3 If all plants were heliotropic & none apheliotropic, one might believe this. Stahl is working at the effects of light on the movements of swarm-spores, & he says he can make them heliotropic or apheliotropic as he likes, if puts some in bright sun & others in darkness & then put them both in diffused light, the sun-ones go to the darkest side & the dark ones to the lighter side.4

July 25th Something or other stopped me finishing. I was rather seedy last night & didn’t appear at the laboratory & this morning Sachs came all the way to see how I was, & drove me to the Labor in his drosky, & was very kind wanting to send me books & red-wine which is here the cure for all evils. I am all square this afternoon though rather too floppy to work. I will talk to Sachs about roots & geotropism. I was very glad to find that Sachs is dead against all the people that find the Descendenz theory in Ray Lamarck, Goethe &c.5 Sachs says that he believes some ferns of the family Marratiaceæ sleep, & some plants of the family in which Canna is. I think in my list I told you that Guiacum officinale looks as if it ought to sleep but seemed to be ill, I thought it was a Leguminosæ, but it is a Zygophyllaceæ & in a lecture Sachs said it slept.6 He says the pale joint in the leaf sheath of grasses is analogous with the movement organs of mimosa &c but he doesn’t know whether they move7   He thinks that we have worked at bloom to some extent from a wrong point of view & he believes that leaves want to keep dry in order that they may keep their stomata open to breathe But why are they so idiotic as to have stomata which shut when they are wet. Perhaps they do that in order to keep their intercellular spaces from being water logged & then found they were stifling themselves & found it a better plan to keep the water off altogether. Or perhaps those leaves produced bloom whose stomata did not shut well in wet. He says that nerves have no bloom & also no stomata. He can give me two references to papers where the number of stomata on the upper & under side of lots of leaves is given, & by seeing about the bloom in these one might make out something.8 T. resupinatum ought to help us, where the stomata differ on the two sides.9 The result:

AL incomplete

CD annotations

1.1 I have … the stalk. 1.3] crossed pencil
1.2 next morning … shut.] scored red crayon; ‘Porliera’ red crayon
2.1 I had … heliotropism] ‘Heliotropism’ blue crayon
2.2 he says … heliotropic. 2.3] double scored red crayon, ‘Theory’ red crayon
2.5 they won’t grow … crystal. 2.10] crossed pencil
2.11 Stahl … lighter side. 2.14] scored red crayon
3.1 Something … said it slept. 3.11] crossed blue crayon
3.8 I think] opening square bracket pencil
3.8 I think … to be ill, 3.10] ‘& Bloom’ red crayon
3.10 Zygophyllaceæ … it slept. 3.11] double scored blue crayon
3.11 leaf sheath of grasses] ‘Bloom’ red crayon circled red crayon
3.13 He thinks … to breathe 3.15] ‘Bloom’ blue crayon
3.15 But why … altogether. 3.18] double scored blue crayon
3.18 Or perhaps … something. 3.22] ‘M. Mer experiments about keeping leaves closed for week under wet’10 pencil
3.22 T. resupinatum … sides. 3.23] double scored red crayon
Top of letter: ‘Bloom & General Theory’ pencil
1
The garden at Würzburg had two Porlieria plants, one planted in the ground and one in a pot; the former usually kept its leaves shut, even in daylight (see letter from Francis Darwin, [after 7 July 1878] and n. 3). CD suggested it should be watered copiously to see whether it then stayed awake (letter to Francis Darwin, 14 July [1878]).
2
CD had asked Francis to put some of these questions about heliotropism to Julius Sachs (see letter to Francis Darwin, 20 [July 1878]).
3
Carl Wilhelm von Nägeli had opposed what he termed the theory of ‘usefulness’ (‘Nützlichkeitstheorie’, that is, CD’s transmutation theory) to his own theory of perfectibility (‘Vervollkommnung’), which posited an inner tendency towards a more complex organisation (Nägeli 1865, p. 29). He argued that CD’s theory could only account for development in functional features whereas his own theory explained changes in what he termed purely morphological characteristics, that is, features with no functional advantage. For more on Nägeli’s theory, see Correspondence vol. 16, letter to J. D. Hooker, 25 December [1868].
4
Ernst Stahl was Julius Sachs’s assistant. Sachs had directed him to study the effect of light on plants; swarm-spores (also known as zoospores) are the motile, swimming spores in non-vascular plants. CD cited Stahl’s short work (Stahl 1878) on the influence of light on swarm-spores in Movement in plants, p. 488 n.
5
CD had acknowledged Jean-Baptiste de Lamarck and Johann Wolfgang von Goethe as advocates of the theory of common descent, although his understanding of Goethe was based on his reading of Isidore Geoffroy Saint-Hilaire (Origin 3d ed., pp. xiii–xiv). John Ray had defined species based on the concept of genealogical lineages (Ray 1686–1704, 1: 40), but CD recognised that Ray was not a transmutationist. ‘Descendenz-theorie’ was the German term most frequently used at this time to refer to evolution, and by some authors, like August Weismann, specifically to refer to Darwinian theory (Weismann 1875a, 1876).
6
Francis evidently intended Marantaceae (the family of arrowroot). In older taxonomic systems, the genus Canna (canna lilies) belonged to the family Marantaceae (Lindley 1853, p. 169); it now belongs to the related family Cannaceae. Francis had, in fact, referred to Ligophyllum Guaiacum, an unknown combination, and noted that while the plant ought to shut, the specimen he observed was very unhealthy (letter from Francis Darwin, [12 July 1878]). Guaiacum officinale is lignum vitae; Zygophyllaceae is the family of caltrop.
7
Sachs was evidently referring to pulvini in festucoid grasses, where the pulvinus consists of a band of cells encircling the leaf sheath, at the apex of the point where the sheath attaches to the node. Pulvini of Mimosa (the genus of sensitive plants) and similar genera are at the base of leaf-stalks or petioles (of leaf, leaflet, and pinna).
8
Throughout 1878, CD and Francis had been investigating the function of bloom on leaves (CD’s ‘Journal’ (Appendix II)). Sachs probably mentioned Czech 1865 and Weiss 1865, as these contain lists of plants with the location and number of their stomata; they were among papers mentioned in Francis’s later publication on the relation between bloom and the distribution of stomata (F. Darwin 1886, p. 102).
9
Trifolium resupinatum is Persian clover.
10
Émile Mer had studied the effects of submersion in water on leaves of various species of plants (Mer 1876; CD’s annotated copy is in DAR 136: 12); he noted that submerged leaves did not produce amylase, but that periodic movements persisted, although less pronounced than usual (ibid., p. 256).

Please cite as “DCP-LETT-11628,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-11628