24. 



SCIENCE OF AGRICULTURE. 



Part II. 



1 571 . If the branch of a tree is dtuated as in the foregoing case of the stem, so as to 

 be partially or periodically immersed in water, it will send out also the same sort of 

 brush-like shoots. 



\m Ihlnchcs or knots, exhibiting a plexus of young shoots (^^. 233 a.) issuing flrom nearly the same point, 

 ' ' ^1 directions, and finally incorporating together oy means of a sortot natural graft, frequently 



ossmg in 



233 



disfigure the branch. These bunches are frequently to be met with ou the branches of the birch-tree, and are 

 knovvn among the peasantry of Scotland by the name of witches' knots. Thoy are occasionetl, like the bun- 

 ches of the stem, by some obstruction in the channel of the sap or proper juice. A peculiar sort of knot or 

 bunch is also formed on the branches of the dog-rose. The - - 



nucleus, which is generally from an inch to an inch and a half 

 in diameter, is covered with a long and winged shag, first of a 

 green and then of a purple color, presenting the appearance 

 of a small bunch of moss. {Jig. 234.) It has been occasioned 

 like that of the stem of the thistle, by the puncture of an 

 insect depositing its eggs in the tender shoot ; for if it is cut 

 open about the month of August, it contains maggots. 

 These anomalies remind us always of that singular disease in 

 the human species, the Plica polonica. 



1573. The bud. The regular developement of 

 the bud is also often prevented by means of the 

 puncture of insects, and converted into a large 

 globular tumor. 



1574. The gall tumor is very often effected by a species of 

 Cynips that lances its piercer into the heart of the bud while 

 yet tender, and penetrates with its saw into the very pith ; in- 

 jecting at the same time a drop of the corroding liquor con- 

 tained in its bag, and then laying its eggs. The bud being thus wounded, and the juices corrupted by 

 the injected poison, the circulation is not only impeded, but a fermentation is induced which burns the 

 contiguous parts and changes their color. The extravasated juice flows round the egg, and is there accu - 

 mulated and converted into a sort of spongy lump which vegetates and augments till it forms what is 

 called a gall. The gall thus formed affords both shelter and nourishment to the voung maggot, which, 

 after being converted into a fly, pierces its enclosure and launches into the open air. The most remark- 

 able of such galls are those produced on the oak-tree, and known in this country by the vulgar name of 

 oak-apples, {fig. 233 b.) The bud of the willow, particularlv salix helix, is apt always to be punctured by 

 insects and converted into a gall. But the conversiou is not always complete ; and in this case the shoot 

 remains dwarfish, and the leaves, which are now protruded from nearly the same point, assume some- 

 thing of the figure of a rose. Hence it has obtained the common name of the rose- willow. The galls of 

 the salvia pomifera, formed in the above manner, are said to be of a very pleasant flavor, and are esteemed 

 a great delicacy in eastern countries. 



1575. The leaves, like the buds, are also frequently chosen for the nidus of in- 

 sects, and disfigured with galls or excrescences. But the most remarkable gall 

 produced on the leaf, and indeed the most remarkable and important of all galls, 

 is that which is so extremely useful in the arts of dyeing and making ink, the nut-gall 

 of the shops. 



1576. The nut-gall is generated on the leaf of a species of oak that grows plentifully in the Levant, and 

 is so well known in commerce as to require no particular description. It is occasioned by the puncture of 

 the Cynips quercifolii, which deposits its egg in the substance of the leaf, by making a small perforation on 

 the under surface. Galls and tumors are to be found on the leaves of many plants ; and indeed almost all 

 leaves are liable to deformities, giving them a bUstered, wrinkled, or curled appearance ; and often pro- 

 ducing disease. 



1577. The excess or deficiency of leaves protruded in a group sometimes constitutes the anomaly, as in 

 the case of the trefoils. 



1578. Sometimes in the natural figure of the leaf itself, as in asparagus officinalis, where they are bristle- 

 shaped ; salsola kaU, awl-shaped; and alium cepa,.in which they are tubular, tapering to a point. 

 But one of the most remarkable anomalies of figure is that which occurs in the leaves of the genus 

 Sarracenia, of which the lower portion is tubular, ascending, and lapproaching to funnel-shaped, or 

 rather pitcher-shaped reversed, with a flattened and concave limb attached by the one side to the orifice of 

 the tube, and constituting the upper portion of the leaf linnseus, who was acquainted with this singula- 

 rity of structure, accounted for it by supposing that it was an institution of nature, meant for the purposef 

 furnishing the plant with a supply of water, which it could thus catch and retain in the leaf. But as some 

 species of the genus do not readily admit water notwithstanding their capacity to retain it, this hypothesis 

 is regarded by Sir J. E. Smith as being extremely doubtful, who accordingly offers a different solution. 



