196 PROCEEDINGS 0? THE AMERICAN ACADEMY 



This is the origin of the tip of sucli a portion as that labelled A" in 

 Figure 1. At B in Figure 18, about 30 /x below the section figured 

 in the preceding, this group of four cells has increased by dividing to 

 form six. The divisions are repeated, and this portion grows vpider 

 until the intervenijig space is traversed and it unites with the main 

 part. 



This fact is significant in indicating that there is no one cell, nor 

 even one region, whence all growth proceeds, but several cells and 

 regions, and these somewhat irregularly placed. 



As we go down, the outer cells, one by one and at first somewhat 

 irregularly, divide into two by a wall parallel with the surface of the 

 frond, until at length we have a layer of smaller cells on each side, a 

 layer of large cells adjoining each of these, and the tubes in the mid- 

 dle. The outside layers on each side may be called the limiting layers 

 (L in Figs. 19 and 20) ; the next layers, of larger and more irregular 

 cells on each side, may be spoken of as the cortical layers (C in 

 Figs. 19 and 20); and the tubes alone represent what later demands 

 attention as the medulla. 



The lower portion of the blade does not increase in complexity for 

 some distance, but widens until the whole width of the blade is occu- 

 pied by the many-layered portion. But in the region where the blade 

 narrows and passes gradually over into the stipe, in what has been 

 spoken of above as the transition place, a new set of structures arises, 

 which are intimately concerned with the thickening of the blade and 

 stipe, and the increase in the complexity of the tissues. The fact 

 that this set of changes originates in this portion shows that the prin- 

 cipal meristematic region is situated here in this species, as it is in 

 others of the same group. 



At the transition place in such specimens the limiting layer is dis- 

 tinct, and consists of rather small cells of nearly equal dimensions in 

 the three principal directions. The cortex has two, or even three, 

 layers of two more or less distinct kinds of cells. Those of the outer 

 layer are like the cells of the simple cortical layer. The inner cells 

 appear in transverse section much like those of the outer layer, except 

 that they are more rounded and lie more separated both from one an- 

 other and from the outer layer, and in the middle layer among the 

 tubes. In longitudinal sections, however, they are seen to be much 

 more elongated than the outer, some of them being five to six times 

 as long as broad. From them, too. are given off short hyphal branches, 

 which extend out laterally from the cells, and protrude into the mi'ldie 

 portion. It was impossible to see the mode of origin of these hyphal 



