Sept.. 1894. SOME NEW BOOKS. 227 



a state of things hitherto quite unsuspected either by meteorologists 

 or theorists on flight. Extreme variabihty seems to be equally charac- 

 teristic of strong and of gentle winds, nor is there any reason to 

 suppose that a still lighter instrument recording still more frequently 

 would not bring to light further inequality. It is quite likely that 

 "the incessant alterations which, it appears, make ' wind,' are due 

 to impulses and changes which are preserved in it, and which die 

 away with considerable slowness." What we should like to know 

 more of is the change (if any) in direction which accompanies the 

 change in velocity. Is the " structure " of the wind mainly undula- 

 tory or mainly vertical ? that is to say, are the variations observed 

 at a fixed point due to the passage of " ripples " or minute 

 " cyclones " ? Upon the answer to this question, it seems as if the 

 future of what Professor Langley calls " Aerodromics " will largely 

 depend. 



R. C. GiLSON. 



Plant Classification. 

 Uebersicht des NATiiRLiCHEN SYSTEMS DER Pflanzen. Zum Gebrauch in 



Vorlesungen fiir Anfanger. ByE.Pfitzer. 8vo. Pp. iv.,36. Heidelberg, 1894' 



Price IS. 

 Professor Pfitzer's " Ubersicht des Natiirlichen Systems der 

 Pflanzen," designed primarily for students attending his course of 

 lectures for beginners, will, we doubt not, be found useful at centres 

 of learning other than Heidelberg. It is a small octavo volume of 

 thirty-six pages, with one side blank for notes, and contains a sketch 

 of plant classification. In a few lines are given the chief charac- 

 teristics of the great groups and divisions, and, a pi'opos of these, we 

 cannot fail to notice the changes which have occurred in this branch 

 of the science during the last few years. Those of nomenclature, 

 though necessitating the loss of well-known terms, indicate an advance, 

 in that real and not imaginary differences are now expressed. Instead 

 of the Phanerogams and Cryptogams of the text-books of not many 

 years ago, we now find plants ranged under the three great groups, 

 Siphonogamae (equals Phanerogams), where the egg, or oosphere, is 

 contained in an ovule and fertilised by the agency of a pollen-tube ; 

 Archegoniatae, where the egg is contained in an arclicgoninm and ferti- 

 lised by an independent cell — the anthei'ozoid, and including (i) the 

 ferns and their allies, the lycopods and equisetums, (2) the mosses 

 and liverworts ; and, thirdly, the Thallophyta, plants which form 

 neither archegonium, ovule, nor pollen-grain, and including the two 

 great divisions of Algae and Fungi. 



Of the two divisions of the Siphonogamae, Angiospermae and 

 Gymnospermae, the latter, in which, though fertilisation takes place by 

 means of a pollen-tube, the egg is contained in a rudimentary arche- 

 gonium, finds a most natural position at the end next the true 

 Archegoniatae. We may also note the tentative subdivision of the 

 Angiosperms into Acrogamae where the pollen-tube reaches the egg 

 by means of the micropyle, and endosperm is formed after fertilisation, 

 and the Chalazogamae, where the mode of entrance is by way of the 

 chalaza, while endosperm is formed before fertilisation. The former 

 include everything except the Australian genus Casiiavina, which 

 constitutes the order Casuarinaceae, and at present the subdivision 

 Chalazogams. In the subdivision of the dicotyledons with free petals 

 (Choripetalae) there has been much alteration from Bentham and 

 Hooker's separation into Thalamiflorae, Disciflorae, and Calyciflorae. 

 Only the last of these remains as one of six, while the whole is enlarged 



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