^XXIV IKTRODUCTION, 





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Species. Tliese tables sliould be so constructed as to contain, under each bractct, or 

 eqnallj indented, two (rarely tliree or more) alternatives as nearly as possible contradic- 

 tory or incompatible witli each other, each alternative referring to another bracket, or 

 having under it another pair of alternatives further indented. The stndent having a 

 plant to determine, -u-ill first take the general table of iS"atural Orders, and examining 

 his plant at each stop to see which alternative agrees with it, will be led on to the 

 Order to which it belongs ; he will then compare it with the detailed character of the 

 Order given in the tcxL If it agrees, he will follow the same course with the table of 

 the genera of that Order, and again with the table of species of the genus. But ni j 

 each case, if he finds that his plant does not agree with the detailed description of the 

 genus or species to which he has thus been referred, he must revert to the beginning 

 and carefully go through every step of the investigation before he can be satisfied. A 

 fresh examination of his spechnen, or of others of the same plant, a critical considera- 

 tion of the meaning of every expression in the characters given, may lead him to detect 

 fiome minute point overlooked or mistaken, and put him into the right way. Species 

 vary within hmits which it is often very difficult to express in words, and it proves 

 often impossible, in framing these analytical tables, so to divide the genera and species, 

 that those which come under one alternative should absolutely exclude the others. 

 In such donbtfid cases both alternatives must be tried before the stiulent can come to 

 the conclusion that his plaat is not contained in the Plora, or tliat it is erroneously 



described. 



24G. In those Floras where analytical tables arc not given, the student is usually 

 guided to the most important or prominent characters of each genus or species, eitlier 

 by a general simimary prefixed to the genera of an Order or to the species of the 

 genus, for all such genera or species ; or by a special summary immediately preceding 

 the detailed description of each genus or species. In the latter case this summary is 

 called a diagnosis. Or sometimes the important characters are only indicated by 

 italicizing them in the detailed description. 



247. It mny also happen that the specimen gathered may present some occasional ' 

 or accidental anomalies peculiar to that single one, or to a very few individuals, which 

 may prevent the species from being at one recognized by its technical characters. It 

 may be nseful liere to point out a few of these anomalies which the botanist may be 

 jnost likely to meet with. For this purpose we may divide them into two classes, viz. : 



1. Aherraf ions from the ordinary type or aj)2)earance of a species for which sojne 

 general cause may lie assigned. 



A bright, light, and open situation, particularly at considerable elevations above the 

 sea, or at high latitudes, without too' much wet or drouglit, tends to increase the size and 

 heigliten the colour of flowers, in proportion to the stature and foliage of the plant. 



Shade, on the contrary, especially if accompanied by richness of soil and sufficient 

 moisture, tends to increase the foliage and draw up the stem, but to diminish the num- 

 ber, size, and colour of the flowers. 



A hot cUmate and dry situation tend to increase the hairs, prickles, and other pro- 

 ductions of the epidermis, to shorten and stiffen the ^iranchesj rendering thorny plants 

 yet more spinou.-^. Moisture in a ricli soil has a contrary cfTect. 



The neighbourhood of the sea, or a saline soil or atmosphere, imparts a thicker ana 

 more succulent consistence to the foliage and almost every part of the plant, and ap- 

 eais not imfrenueuLly to enable plants usually annual to live through the whiter, 

 'lowers in a maritime variety are often much fewer, but not smaller. 



The luxuriance of plants growing in a rich soil, and the dwarf >t unted character of 

 those crowded in poor soils, are too well known to need particularizing. It is al^o an 

 everyday observation how gradually the specimens of a species become dwarf and 

 stunted as we advance into the cold damp regions of the summits of high mountain- 

 ranges, or into high northern latitudes; and yet it is frequently from the want of at- 

 tention to these circumstaiices that numbers of false species have been added to our 

 Enimierations and Floras. Luxuriance entails not oidy iucrease of size to tlie whole ^ 

 plant, or of particular parts, but increase of number in branches, in leaves, or leaflets 

 of a compound leaf ; or it may diminish the hairiness of the plant^ induce thorns to 

 grow out into branches, etc. 



