182 MINXESOTA BOTAXICAL STUDIES. 



dark brownish spots on their upper surfaces. Tlie older fronds 

 are irregularly perforated with circular holes and are more or 

 less torn at the edges (Fig. 3). When young the fronds are en- 

 tire (Fig. 2). As the plant continues its growth the stem pushes 

 up through the center of the frond, forming a new growing 

 point which elongates into an internode and soon forms another 

 frond at its summit. The old frond then falls off, leaving an 

 annulate marking on the stipe (Figs. 5, 6). There is often, espe- 

 cially in the youngest portions of the oldest plants, a formation of 

 two growing points at the center of each lamina giving rise to 

 a dichotomv of the stipe (Fig. 5). Irregularity in the develop- 

 ment of these growing points gives rise to an irregular system 

 of branching (Fig. 5). Growing points are not confined to the 

 bases of laminae but may be found occasionally arising from other 

 portions of the stipe. There are often a group of small branches 

 formed upon the holdfast (Fig. i). The latter is a disc-like body 

 arising as an expanded portion of the stipe at its base and usu- 

 ally concave below (Fig. 22). 



Upon the lower surface of the fronds especially the larger 

 ones, can be seen distinct radial striations caused by small ridges 

 running from the region near the stipe toward the edge of the 

 frond (Fig. 4). These mark the course of bundles of elongated 

 narrow filaments (see below). The nemathecia which bear the 

 tetraspores are found only on the lower surface of the frond. 

 They occur more abundantly on the larger fronds and are often 

 so numerous as to almost completely cover the under surface. 

 They form small wart-like bodies of whitish color and gelatin- 

 ous consistency. 



Minute Anatomy. — As is well know^n the thallus of the red 

 seaweed is a group of dichotomously branching filaments whose 

 fused branches form tissue-like areas. In Constantinea this 

 method of branching can readil}' be seen in the frond but par- 

 ticularly so in the cortical area of the stipe (Fig. 18). 



Frond. — A cross section of the frond shows three areas of 

 cells : {a) a central layer of loosely woven filaments ; ((5) an in- 

 termediate layer of large approximately spherical cells stuffed 

 with starch granules (Fig. 7_) ; {c) a cortical la3^er of pseudo-pa- 

 renchymatous cells. 



a. There are in the central area filaments of large cells, 

 usually slightly elongated, often stretching across the frond and 

 perpendicular to the frond surface (Fig. 11). These cells ap- 



