ACT 



ACU 



without cotyledons; for, in that case, 

 cuscuta would be acotyledonous. On the 

 contrary, it is an embryo which does not 

 germinate from two fixed invariable 

 points, namely, the plumule and the ra- 

 dicle, but indifferently from any point of 

 the surface, as in some Araceae, and in 

 all flowerless plants. 



ACOU'STICS {uKovw, to hear). The 

 science of sound ; the laws which regu- 

 late the vibrations of air, or of other 

 media, in their relation to the organ of 

 hearing. 



A'CRITA (afcptTo?, indistinct). A pri- 

 mary division of the Animal Kingdom, 

 composed of the lowest classes of tlie 

 radiate animals, and characterized by an 

 indistinct, diffused, or molecular condi- 

 tion of the nervous system, and by the 

 absence of distinct parietes to the ali- 

 mentary canal. To this division belong 

 the medusa, the polype, &c. These are 

 the cryptoneura of Rudolphi, the protozoa 

 and onzoa of others. See Nematoneura. 



A'CROGEN (ciKpor, extreme, jeivofxai, 

 to grow). Point-grower ; the classical name 

 of a plant which grows only at its point, 

 or upper extremity, as a fern tree, and is 

 thus distinguished from an endogen on 

 the one hand, and an exogen on the other. 



ACRO'LEINE {aK^o^, extreme, eXaiov, 

 oil). A substance of a very pungent 

 odour given off by oils and fats when 

 boiling at a high temperature, and pro- 

 duced in large quantity by the distillation 

 of pure glycerin. 



ACRO'NYCHAL (aKpor, extreme, vi,^, 

 night). A Greek term denoting the ex- 

 tremities, or the beginning and the end, 

 of the night. Hence, a star is said to be 

 acronychal, or to rise acronychally, when 

 it rises at sunset, and consequently sets 

 at sunrise. 



A'CROSPIRE (afcpoc, extreme, crirelpa, 

 a spire). That part of a germinating em- 

 bryo which botanists call the plumula, 

 and which bears the cotyledons. It has 

 a curved form, and makes its appearance 

 at the extremity of tlie seed, 



ACTERAPMIN. A star of the third 

 magnitude, in the left shoulder of Ce- 

 pheus, marked a. 



ACTINE'NCHYMA {cIktIv, a ray of 

 light, eyx^l^oiy an infusion). A term ap- 

 plied by some botanical writers to the 

 stellate, or star-shaped variety of sphe- 

 roidal cellular tissue. 



ACTPNIA (^Krlv. a ray of light). A 

 form of polype, in which the mouth oc- 

 cupies the centre of the upper surface, 

 and is surrounded by tentacula, which 

 7 



radiate from the centre, like the petals of 

 a flower. Hence, the genus has acquired 

 the names of animal flowers, sea ane- 

 mones, &c. From the fibrous character 

 which the substance of their bodies as- 

 sumes, they have been named by zooJo- 

 gists fleshy polyps. 



ACTINO'CEROS {^iktIv, a ray, Kepaf, 

 a horn). A generic term, signifying the 

 radiated disposition of the horns or 

 feelers of animals. 



ACTI'NOLITE {dKrlv, a ray of light, 

 \i0oi, a stone). A green-coloured mine- 

 ral, forming a variety of hornblende, and 

 usually occurring in fascicular crystals, 

 which are arranged in the form of rays. 

 It is found chiefly in primitive districts 

 with a magnesian basis. There are three 

 varieties, — the crystallized, the asbestous, 

 and the glassy. 



Actinolite slate. A hornblendic rock, 

 composed of actinolite and felspar, and 

 easily recognized by its light-green colour, 

 and the elongated form or fibrous appear- 

 ance of its principal constituent mineral. 



ACTINO'METER(<iKTiv, arayof light, 

 lx€Tf>ov, a measure). An instrument for 

 measuring the intensity of light. This 

 instrument indicates the force of sun- 

 shine at the Cape of Good Hope as 48° 

 75', while ordinary sunshine in England 

 is only from 25° to 30°. 



ACTION. The motion which one body 

 produces, or endeavours to produce, in 

 another. Mechanical action is exerted 

 by percussion or by pressure ; and, in 

 either case, the force exerted by the act- 

 ing body is repelled in an equal degree 

 by the body on which it acts : the stroke 

 of the hammer on a nail, acts upon the 

 former equally as on the latter. In all such 

 cases, the counteracting force is termed 

 re-action; and it is a law in Physics that 

 "action and re-action are equal." 



ACTIVITY, SPHERE OF. The space 

 within which the action of a body pro- 

 duces a sensible effect. 



A'CUBENE. A star of the fourth 

 magnitude, in the southern claw of Can- 

 cer, marked a. 



ACULEA'TA {aculeus, a sting). A 

 group of Hymenopterous insects, in which 

 the abdomen of the females and neuters 

 is armed with a sting connected with a 

 poison reservoir. The antennae are twelve 

 in number in the female, and thirteen in 

 the male. 



ACU'LEATE {aculeus, a prickle). 

 Prickly ; a term applied, in botany, to a 

 surface covered with prickles, as the stem 

 of the rose. 



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