ZOOLOGY AND r>OTANY, MICKOSCOPY, E'lT. 337 



measuring the humidity of the air, \Yhich would seem to be of cousiderable 

 importance in causing increase of growth. The method of measurement 

 appears to be simple and satisfactory, and therefore suitable to further 

 observations along the same lines. S. G. 



CRYPTOGAMS. 



Pteridophyta. 



External Morpholog'y of the Stems of Calamites, with a Revision 

 of the British Species of Calamophloios and Dictyocalamites of Upper 

 Carboniferous Ag^e. — ^E. A. Newell Arber and F. AV. Lawfield 

 {Journ. Liiiii. Soc, 1920, 44, 507-30, 3 pis.). An account of the 

 external features of Calamites stems. As compared with fossilized pith 

 casts, the external casts or impressions are rare and have often been 

 confused with pith casts. Tiie features of pith casts are their nodes and 

 their ridged and grooved internodes ; and there may be infranodal canals 

 and branch scars. Submedullarv casts are frequent, representing a 

 region within the secondary wood ; they are characterized by very broad 

 ribs (medullary rays) and the absence of infranodal scars. External 

 casts show the following characters : nodes, internodes (smooth, striated, 

 etc.), leaf scars, branch scars, root scars, which are discussed in detail. 

 'An attempt is made to correlate the specimens of British Calamites which. 

 show external surfaces with their corresponding pith casts. And this is 

 followed by a systematic revision of the British species of Calamophloios 

 and Didyocalamites. A. Gepp. 



Physiological Anatomy of Xerophytic Selaginellas.— J. C. Th. 

 Uphof {New Phijtologisf, 1920, 19, 101-31, 12 figs.). Six p.c. of the 

 species of Selaginella are xerophytic. In the xerophytic species the 

 dorsal and ventral leaves are closely approximated, and exhibit a striking 

 similarity of size— a matter of much importance, since the ventral leaves 

 contain but few chloroplasts, the work of photosynthesis devolving upon 

 the comparatively large dorsal leaves. The hygrophytic species on the 

 other hand have their leaves spaced out, and show a marked difference 

 in size between the dorsal and ventral leaves. There are three groups 

 of xerophytic Selaginellas : — 1. Plants vdth vertical leaves of the same 

 shape and size ; the apex of each leaf ends in a long awn containing no 

 chloroplasts ; the anatomical construction is sclerotic. 2. Plants with 

 slender, wiry, trailing stems, spreading over the ground or hanging from 

 rocks and sometimes trees. 3. Plants with a spreading habit, often 

 forming a flat, dense and close rosette, rolling into a cluster-ball during 

 drought. In the first group the erect leaves form a protection against 

 intense insolation. The growing points of the stem receive the sun's rays 

 directly, but the apical bundle of awns reflects the light entirely. The 

 epidermis, hypodermis and outer cortex are thick-walled and heavily 

 impregnated with suberin. The elements of the protoxylem and 

 metaxylem are much narrower than in hygrophytic species. Some 

 species are petrophilous, some are psammophilous. The second group 

 comprises straggling plants, less sclerotic than the previous group, but 

 very thick-walled. Some have a red pigment in very exposed stems ; 



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