Description and physiological botany / [J.S. Henslow].

  • John Stevens Henslow
Date:
1839
Catalogue details

Licence: Public Domain Mark

Credit: Description and physiological botany / [J.S. Henslow]. Source: Wellcome Collection.

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    (monocarpeans) is greatest in temperate regions^ and diminishes both towards the equator and poles. Many local circumstances produce remarkable mo- difications in the relative proportions between the species of different classes and orders, in regions under the same parallels of latitude. Thus for instance, ceteris paribus, the cryptogamic tribes flourish most in moist regions. The places best adapted to the growth of ferns are the islands in tropical climates, in some of which, as in St. Helena, one half the flora is composed of them. It is remarkable that in this respect, and as regards the existence of arborescent species in this order, the ancient flora of our coal-fields, appears to approximate very closely to that of islands situate in the midst of an extended ocean and in low latitudes. The same causes which appear favourable to the in- crease of cryptogamic species, seem also to produce a diminution in the proportions which dicotyledons bear to monocotyledons. Other relations of consi- derable interest have been pointed out between the species of different orders, occurring in different re- gions ; but we cannot enter into the minutice of their details, our object being rather to present the reader with the principles on which such investigations depend, than to acquaint him with the partial results which have hitherto been deduced from them; several of which must doubtless be greatly modified hereafter, considering the little knowledge we at present possess of the floras of many parts of the world. The following table exhibits a few of those results which appear to have been most satisfactorily esta- blished. It gives the relative proportion which ten well-defined orders, or families of plants, bear to the whole of the phanerogamic tribes in the torrid, tem- perate, and frigid zones respectively, and shows us in which they occur in the greatest relative abundance, decreasing as we recede from that zone towards the others.
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