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 25^th 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: