82 COMPOUND ORGANS OF PLANTS. 



represents the Alsophila armata, a tree-fern of Brazil.* As the 

 lower fronds decay and drop off, the young upper fronds gra- 

 dually unroll and become pendent, and in this manner con- 

 tinual additions are made to the height of the stem, and the 

 growing point is carried upwards, the stem increasing in alti- 

 tude, but not in diameter. 



113. It is for this reason that these plants are called Acro- 

 gens (axpo; a summit, and yiwdnv to produce) or summit- 

 growers, because they increase by additions of cellular and 

 vascular matter to their extremities, and have no provisions 

 for any subsequent increase in diameter. 



114. Ferns, in their infancy, resemble the adult liverworts, 

 having a green, flat, pro-embryo like the thallus of the frondose 

 liverworts. This pro-embryo or pro-thallus afterwards gives 

 rise to the proper stem and frond of the fern. In Fig. 34, s is 



the spore of a fern, the Pteris longifolia, sprouting and giving off 

 a rootlike process r, and a flat cellular expansion p, called the 

 pro-thallus or pro-embryo. It is on this expansion that the 

 antheridia and pistillidia occur, and the former having fulfilled 



* Richard's "Precis de Botanique," pt. i., P.oi-is, 1852. 



