288 
PHYSIOLOGY. 
BOOK II. 
woody matter downwards. Some valuable observations upon 
this point have been made by Mohl, who has, however, been 
able only to investigate the anatomical condition of Tree Fern 
stems, without studying their mode of growth. Lycopodiaceae 
equally increase by simple addition to the point; and, as this 
seems also to be the plan upon which developement takes 
place in other cryptogamic plants, I have proposed the term 
Acrogens, to distinguish the latter from Exogens and En- 
dogens. 
When leaves are no longer formed, but growth takes place 
by an irregular expansion of cellular tissue in various direc- 
tions, the preceding rules are departed from, and nothing 
being left of the vegetable fabric except the horizontal order 
of growth, a stem ceases to appear, and a plant becomes an 
unsymmetrical body, either consisting of solid masses in- 
creasing in all directions, or of filamentous matter multiplying 
itself by internal septation at the elongating apex. 
