399 



which some naturalists have been disposed to compare to animal in- 

 stinct. Mr. Knight examines whether this comparison has any foun- 

 dation. 



He remarks, that when trees which require moisture are near to 

 water, by far the greatest part of their roots extend themselves .in 

 that direction ; but when those to which a dry soil is more congenial 

 are placed in the same situation, their roots take an opposite course. 

 When a tree is placed upon a wall, at a distance from the ground, 

 its powers appear directed exclusively to one object ; its roots are 

 extended with well-directed efforts to reach the soil beneath, while 

 its branches are in the mean time stinted in their growth ; but as 

 soon as the roots have entered the soil, the branches then grow with 

 vigour and rapidity. 



Mr. Knight sowed some seeds of the common carrot and parsnip 

 in light poor loam, under which, at the depth of eighteen inches, he 

 had placed a stratum of very rich mould, incorporated with fresh 

 vegetable matter. The plants grew feebly till the end of summer, 

 but then assumed a very luxuriant growth ; and when the roots were 

 examined, they were found nearly cylindrical, with very few fibres 

 emitted into the superficial poor soil, whilst the rich mould beneath 

 was filled with fibres. 



When the strata of soil were reversed, the roots were found to 

 have formed accordingly, ramifying, in an unusual manner, into the 

 superficial rich soil, but emitting few o their fibres into the subjacent 

 stratum. 



When mould contained in an inverted garden-pot was so supported 

 by frame-work that its under surface was exposed to the air, the ra- 

 dicles of garden-beans in contact with this surface extended horizon- 

 tally along it, and in a few days emitted many fibrous roots upwards 

 into the mould ; but they appear to the author to be guided by laws 

 far more simple than animal instinct. 



For the explanation of these laws he refers to the known powers 

 of the alburnum of the stem and branches, by which the sap is pro- 

 pelled, without assistance from the roots, in such proportion as each 

 has power to employ ; that which is exposed to the light attracting 

 to itself a larger portion of the ascending sap, for the formation of 

 leaves and of vigorous annual shoots, while those that are shaded 

 are comparatively languid and unhealthy. In the same manner, the 

 author is of opinion, that the stimulus of proper food and moisture 

 in the soil upon the bark of the root may give ability to that organ 

 to attract and employ a larger portion of the descending current of 

 sap. Those roots of a tree upon a wall, that are the worst supplied 

 with moisture, will be first affected with drought, and their points 

 will become rigid and cease to elongate, and the current of descend- 

 ing sap will be employed in the elongation of those more favourably 

 circumstanced, and the tree will appear to have adopted as wise a 

 plan as it could have done if it had possessed the feeling and in- 

 stinctive powers of animal life. 



The cylindrical roots of the carrots and parsnips, in the first experi- 



