14 MINNESOTA BOTANICAL STUDIES. 



ing this a cortex composed of almost unbranched filaments 

 which run outwards from the medulla. At the extreme apex 

 of the frond these cortical filaments are almost cylindrical 

 throughout. Each one has a conical apical cell, and is evi- 

 dently in active growth. A very short distance from the apex 

 of the frond the distal cells of each filament are seen to have 

 become nearly spherical, giving that portion of the filament a 

 moniliform aspect, and thus the assimilating area is established. 

 Apparently after the distal cells of a filament become thus 

 matured it undergoes no further growth by cell division, but 

 the proximal cells may increase very considerably in length. 

 Soon secondary cortical filaments appear in the inner cortical 

 region, arising by the proliferation of the primary cortical fila- 

 ments. A secondary filament generally arises as a lateral out- 

 growth from the distal end of one of the cells of a primary 

 filament. In all respects, except their origin, these secondary 

 filaments exactly resemble the primary ones. They grow out 

 among the latter for a time, and eventually mature, thus in- 

 creasing the number of assimilating filaments. The secondary 

 filaments may in turn give rise by proliferation to other similar 

 filaments. Branching of the cortical filaments has already 

 commenced in the lower part of the tip shown in Fig. 2. A 

 few millimeters farther from the tip, branching will have gone 

 much farther, and each of the original cortical filaments will 

 have developed into a complicated branch system, as in Fig. j, 

 which shows two cells of a primary medullary filament, each of 

 which bears a system of cortical filaments which has developed 

 by the secondary branching of one simple cortical filament. 



At about this stage in the growth of the frond, another kind 

 of filament makes its appearance. Lateral outgrowths arise 

 from the cells of the inner cortex, usually from their proximal 

 portions (S, Figs, j, j). These outgrowths develop into 

 cylindrical, almost simple filaments of much smaller diameter 

 than the cortical filaments. The first of these filaments to 

 appear in any cortical branch system is generally formed as an 

 outgrowth from the proximal part of the basal cell of the 

 system. These filaments are produced successively from the 

 cells throughout the inner cortical region, and eventually the 

 same cortical cell may give rise to several of them. They take 

 an irregular course towards the medullary part of the frond, 

 and eventually grow in among the large medullary filaments, 



