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, Avithout 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 cr^^togamic plants, I 

 have proposed the term Acrogens, to distinguish the latter 

 from Exogens and Endogens. 



11 3 



