352 BOTANY OF THE LIVING PLANT 



approach to a certainty. For they are parts of the gametophyte or haploid 

 phase, not of the diploid sporophyte, and there is reason to believe that those 

 two somatic phases have been distinct throughout Descent (Chapter XXX.)- 

 Further, any strict homology between the leaves of Vascular Plants and the 

 leaf-like' parts seen in highly organised Algae must be held as still more doubt- 

 ful (Chapters XXI. -XXIII.). Before any such comparisons can be accepted 

 as having strict morphological value, the argument must be based not on 

 general preconceptions or surmises, but on the demonstration of similar 

 evolutionary sequences, among- organisms nearly related to one another. 

 Hitherto this has not been done. 



The truth seems to be (i) that in plants at large, whether in the sporophyte 

 phase or in the gametophyte, advantage has been taken of the evolutionary 

 development of lateral flattened surfaces, and their expansion as organs of 

 photosynthesis; (2) that such development may have arisen either in the 

 diploid or in the haploid soma : and (3) that it may have arisen independently 

 along a plurality of evolutionary lines. In other words, that the development of 

 the category of leaves has been widely polyphyletic. 



Minor appendages, in the form of Hairs and Emergences are pro- 

 duced irregularly, and often in large numbers, scattered over the sur- 

 faces of the shoot. They may appear at any point on axis or leaf, and 

 also on the surface of roots, as root-hairs (Figs. 56, 57, 60, 61, 68). 

 The Hair is usually defined as a product of the epidermis only : but 

 the Emergence as involving also the subjacent tissue. This is, how- 

 ever, an arbitrary distinction, though it separates roughly the more 

 delicate from the coarser. As a morphological category hairs and 

 emergences take a quite subsidiary place, and do not rank equally in 

 position or constancy with the leading categories of axis, rachis, 

 pinna, or pinnule. 



The sporangia have been described for the Higher Flowering Plants 

 in Chapters XV. and XVI. : those of Pteridophyta and Bryophyta in 

 Chapters XXX. to XXXII. Our question will be, what relation do 

 sporangia bear to the categories of the vegetative parts on which they 

 may be seated. As regards the Higher Plants their origin has been from 

 time to time referred to Metamorphosis of various parts of their 

 vegetative system. But Von Goebel, in 1881, laid it down, on the 

 basis of wide comparison with those plants lower in the Scale of 

 Descent, that Sporangia are organs sui generis, as much as are 

 shoots and roots : and that they are not referable, through metamorphosis, 

 to any other category of parts, whatever their position relatively to these. 

 That view may be generally adopted, and recent palaeontological 

 discoveries have tended to confirm it. For instance the Bryophyta 

 with their leafless sporogonia, have been traced back to the Carboni- 

 ferous Period, while the Psilophytales, including the leafless Hornea 



