AIR 



AJU 



stant and equal stream ; whereas, in the 

 common squirting engine, the stream is 

 discontinued between the several strokes. 

 Other water-engines are furnished with 

 vessels of this kind. 



A.I it-vessels, in botany, are certain ca- 

 nals or ducts, whereby a kind of absorp- 

 tion and respiration is effected in vegeta- 

 ble bodies. 



Air-vessels have been distinguished 

 from sap-vessels ; the former being sup- 

 posed to correspond to the trachea and 

 lungs of animals ; the latter to their lac- 

 teals and blood-vessels. 



Dr. Grew, in an inquiry into the motion 

 and cause of the air in v egetables, shews, 

 that it enters them various ways, not only 

 by the trunk, leaves, and other parts 

 above ground, but at the root. For the 

 reception, as well as expulsion of the air, 

 the pores are so very large in the trunks 

 of some plants, as in the better sort of 

 thick walking-canes, diat they are visible 

 to a good eye without a glass ; but with a 

 glass, the cane seems as if it were stuck 

 full of large pin-holes, resembling the 

 pores of the skin in the ends of the fin- 

 gers, and ball of the hand. In the leaves 

 of the pine, through a glass, they make 

 an elegant shew, standing almost exactly 

 in rank and file throughout the length of 

 the leaves. But though the air enters in 

 partly at the trunk, and also at other 

 parts, especially in some plants, yet its 

 chief admission is at the root: much as, 

 in animals, some part of the air may con - 

 tinually pass into the body and blood by 

 the pores of the skin ; but the chief 

 draught is at the mouth. If the chief en- 

 trance of the air were at the trunk, before 

 it could be mixed with the sap in the root, 

 it must descend; and so move not only con- 

 trary to its own nature, but in a contrary 

 course to the sap : whereas, by its recep- 

 tion at the root, and its transition from 

 thence, it has a more natural and easy 

 motion of ascent. The same fact is far- 

 ther deduced from the fineness and small- 

 ness of the diametral apertures in the 

 trunk, in comparison of those in the root, 

 which nature has plainly designed for the 

 separation of the air from the sap, after 

 they are bothtogetherreceivedinto them. 



Air-vessels are found in the leaves of 

 all plants, and are even discoverable in 

 many without the help of glasses ; for, 

 upon breaking the stalk or chief fibres of 

 a leaf, the likeness of a fine woolly sub- 

 stance, or rather of curious small cob- 

 webs, may be seen to hang at both the 

 broken ends. This is taken notice of, not 

 only in some few plants, as in scabious, 



where it is most visible : but may also be 

 seen more or less in most others, if the 

 leaves be very tenderly broken. This 

 wool is really a skein of air-vessels, or 

 rather of the fibres of the air-vessels, 

 loosed from their spiral position, and so 

 drawn out in length. 



A IRA, hair-grass, in botany, a genus of 

 the Triandria Digynia class and order, 

 and of the natural order of Grasses. 

 There are twenty-five species, some of 

 which have awns, and others have none. 

 The A. aquatica, water hair-grass, gene- 

 rally grows in the margin of pools and 

 watery places, running into the water to 

 a considerable distance, and is known by 

 the purple or bluish colour of the panni- 

 cles, and sweet taste of the flowers. It is 

 a perennial, and flowers in May and June. 

 To this grass has been attributed the 

 sweetness of Cottenham cheese, and the 

 fineness of Cambridge butter. The A. 

 caepitosa, or tufty-hair grass, grows in 

 moist meadows and woods, is perennial, 

 it flowers in June and July, sometimes 

 trailing on the ground to the length of 

 several feet, and the panicle exhibiting a 

 beautiful silky appearance : cows, goats, 

 and swine eat it, but horses are not fond 

 of it. It is the roughest and coarsest grass 

 that grows in pastures or meadows, and 

 is called by the common people hassocks, 

 rough-caps ; and bull's faces. To get rid 

 of it, the land should be first drained, 

 and the tufts of the noxious weeds pared 

 off and burnt The ashes yield a good 

 manure. The A. flexuosa, or waved 

 mountain grass, is the principal grass on 

 Banstead Downs, and the Mendip Hills 

 It is difficult of cultivation. 



AITONIA, in botany, so called from 

 Mr. Alton, his Majesty's late gardener at 

 Kew, a genus of the Monadelphia Octan- 

 dria class and order, and of the natural 

 order of Columnifera. There is but one 

 species, viz. A. capensis, found at the 

 Cape of Thunberg. It has a shrubby 

 stalk, six feet high, and a fruit resembling 

 that of the winter cherry. With us it is 

 of slow growth, and seldom exceeds 

 three feet in height. At a sufficient age 

 it produces flowers and fruit through the 

 greatest part of the year. 



AJUGA, bugle, in" botany, a genus of 

 the Didynamia Gymnospermia class of 

 plants : the flower is monopetalous andrin- 

 gent ; the upper lip being small and bifid ; 

 the lower one large and trifid : there is 

 no pericarpium : the seeds are contained 

 in the cup of the flower, and are four in 

 number. There are 10 species. The 

 species native in the United States are 7 



