BOOK II. GENERAL CONSIDERATIONS. 245 
cavity ; but, from a specimen of a bamboo in my possession, I 
incline to think that the lower part of grass stems does some- 
times become filled up with solid matter. 
Upon one or other of the two plans now explained are all 
flowering plants developed ; but in flowerless plants it is dif- 
ferent. In arborescent ferns the stem consists of a cylinder 
of hard sinuous plates, connected by parenchyma, and sur- 
rounding an axis usually hollow, but sometimes filled up with 
solid matter. It would seem, in these plants, as if the stem 
consisted of a mere adhesion of the petioles of the leaves in a 
single row, and that the stem simply lengthens at the point 
without transmitting woody matter downwards. Some valu- 
able observations upon this point have been made by Mohl, 
who has, however, been able only to investigate the anatomi- 
cal condition of tree fern stems, without studying their mode 
of growth. Lycopodiaceae equally increase by simple addi- 
tion to the point ; and as this seems also to be the plan upon 
which development takes place in other cryptogamic plants, I 
have proposed the term Acrogens, to distinguish the latter 
from Exogens and Endogens. 
11 a 
