464? Botany, 



of Edinburgh, and published in JamiesorCs Journal for April. According to 

 Dr. Darwin the radicle is stimulated by moisture, and elongates itself where 

 it is most excited; according to Mr. Knight the radicle obeys the laws of 

 gravitation. Mr. Johnson found that the radicle could be made to grow 

 either downwards or upwards, according to the seed's being placed on the 

 upper or the under surface of a mass of moistened earth, a result with which 

 the experience of every gardener renders him familiar. Had Dr. Darwin, 

 instead of the word moisture, used the term moist soil, his theory would 

 have been unanswerable ; as it stands, it is still by far the nearest to the 

 truth. The fallacy of Mr. Knight's theor)', which was adopted by Sir Hum- 

 phry Davy in the earlier editions of his Agricultural Chemistry, is rendered 

 sufficiently obvious by Mr. Johnson's experiments ; but it had ceased to be 

 in repute for a number of years Mr. Johnson agrees with Dr. Thompson 

 and M. Aubert Du Petit-Thouars in ascribing this power of the vegetable 

 to the vital principle. — Cond. 



The Sun-Flower. — I was somewhat surprised to find the following pas- 

 sage in Drummond's First Steps to Botany, an excellent little work : — 

 " Leaves always turn to the light, and some flowers regularly follow the 

 sun, facing him when he rises, and also when he sets. It is strange that the 

 sun-flower is so generally supposed to possess this property, since the 

 slightest observation is sufficient to prove its fallacy. Gerarde detected the 

 error so long ago as 1597." He then quotes the passage from Gerarde's 

 Herhal, p. 614., in support of the above assertion, wherein Gerarde says, he 

 could never observe the property, although he had endeavoured to find out 

 the truth of it. Last year 1 had fifteen sun-flowers in my little garden, and 

 it was a constant source of amusement to observe the flowers following so 

 regularly as they did the course of the sun. In the evening when we 

 retired they all looked towards the setting sun, but in the morning all were 

 turned the contrary way, to be ready to meet the first beams of the great 

 luminary of the day; and this continued daily until the flowers faded, and 

 the work of fructification was over, when the sensibility to the light seemed 

 to be lost, and the head remained fixed. The popular reason for the name 

 of the flower is this very property, and it is rather unaccountable how it 

 should hate escaped the notice of Dr. Drummond. 



Since I wrote the above I found the following passage in Smith's Intro- 

 duction to Botany, p. 159. ed. 4. : — " Nor is the effect of light peculiar to 

 leaves alone. Many flowers are equally sensible to it, especially the com- 

 pound radiated ones, as the daisy, sun-flower, marigold, &c. The stately 

 annual sun-flower, Helianthus annuus, displays this phenomenon more con- 

 spicuously on account of its size, but many of the tribe have greater sensi- 

 bility to light. Its stem is compressed, in some degree, to facilitate the 

 movement of the flower, which, after following the sun all day, returns after 

 sun-set to the east, by its natural elasticity to meet his beams in the morn- 

 ing. Dr. Hales thought the heat of the sun, by contracting the stem on one 

 side, occasioned the flower to incline that way; but if so, it could scarcely 

 return completely at night. There can be no doubt, from the observation 

 of other similar flowers, that the impression is made on their radiated 

 florets, which act as wings, and seem contrived chiefly for that purpose, 

 being frequently destitute of any other use." How is this difference in the 

 two authors to be accounted for? — JD. Stock. Bungay, July 28* 1828. 



Sloughing of Plants. — The power of sloughing has been denied to plants 

 without good reason ; for at a certain time of the year the leaves of a plant 

 decay and fall off": so do ripe fruit. If a shrub, when in leaf, be planted, 

 and any part of it die, the leaves formerly expanded will adhere to it, but 

 those on the living branches will wither, fall off", and give place to others. 

 — {MS Lectures of Sir J. E. Smith.) 



