56 MORPHOLOGICAL DEVELOPMENT. 



Hence this peculiarity which gives their name to the Aerogel 

 now called Archegoniates, is a natural accompaniment of tl 

 low degree of specialization reached in them. And that it 

 an incidental and not a necessary peculiarity, is demonstratec 

 by two converse facts. On the one hand, in those higher 

 Acrogens which, like the tree-ferns, lift large masses of 

 foliage into the air, there is just as decided a transverse ex- 

 pansion of the axis as in dicotyledonous trees. On the other 

 hand, in those Dicotyledons which, like the common Dodder, 

 gain support and nutriment from the surfaces over which 

 they creep, there is no more lateral expansion of the axis 

 than is habitual among Acrogens or Archegoniates. Con- 

 cluding, as we are thus fully justified in doing, that the 

 lateral expansion accompanying longitudinal extension, which 

 is a general characteristic of Phanerogams as distinguished 

 from Archegoniates, is nothing more than a concomitant of 

 their usually- vertical growth ; * let us now go on to consider 

 how vertical growth originates, and what are the structural 

 changes it involves. 



§ 193. Plants depend for their prosperity mainly on air 

 and light : they dwindle where they are smothered, and thrive 

 where they can expand their leaves into free space and sun- 

 shine. Those kinds which assume prone positions, conse- 

 quently labour under disadvantages in being habitually inter- 

 fered with by one another — they are mutually shaded and 



* I am indebted to Dr. Hooker for pointing out further facts supporting 

 this view. In his Flora Antarctica, he describes the genus Lessonia (see 

 Fig. 37), and especially L. ovata, as having a mode of growth simulating that 

 of the dicotyledonous trees, not only in general form but in internal struc- 

 ture. The tall vertical stem thickens as it grows, by the periodical addition 

 of layers to its periphery. That even Thallophytes should thus, under 

 certain conditions, present a transversely-increasing axis, shows that there 

 is nothing absolutely characteristic of Phanerogams in their habit of 

 stem-thickening. Mr. Tansley gives me further verification by the state- 

 ment that "it is also now certain that members of the Equisetinem and 

 Zycopodinece, as well as some Ferns which flourished in Carboniferous times, 

 had secondary thickening in their stems quite comparable to that of modern 

 Dicotyledonous trees." 



