308 Pub. Paget Sound Biol. Sta. Vol, 2, No. 52 



as many which have a variable number of ribs in a single frond as those 

 which have a constant number. It is conjectured that if no part of the 

 frond wore away the complexity of venation might be still further aug- 

 mented. 



The typical frond has a midrib with two ribs on each side. The cor- 

 responding ones on the sides are approximately the same distance from 

 the midrib. In the variants a rather common feature is the absence of a 

 midrib. Thus we have the 4-ribbed plant (Figs. 1 and 2). In no case was 

 a 4-ribbed one found which had a rib in the middle. Of the 6-ribbed 

 fronds several groupings of ribs were found. One group seems to be the 

 result of the separation of one of the outside ribs into two, thus giving 

 two on one side of the midrib and three on the other (Fig. 6). Another 

 plant has a midrib with one on one side and four on the other, apparently 

 due to the failure of the rib on one side to grow, and to the splitting of the 

 2 ribs on the other side into 2 each (Fig, 8). 



A third abnormal form with 6 ribs is characterized by the complete 

 absence of the midrib, and by the presence of two groups of 3 ribs each 

 growing symmetrically at equal distances from the middle of the blade 

 (Fig. 7). The 3 ribs of each group are close together in this. The first 

 and third of each group are not as far apart as the two side ribs normally 

 would be, and the one of each set nearest the center of the frond is stronger 

 than the other two. At the junction of the stipe and blade the two outer 

 ribs of each group almost coalesce, giving the appearance of four ribs 

 arising from the stipe tissue. A fourth variation with 6 ribs has a mid- 

 rib with two normal ribs on one side of it and a group of three on the 

 other (Fig. 5). 



All the fronds having more than 6 ribs show variation in the number 

 thruout the length of the blades. This lack of constancy is also present in 

 the 4 and 5-ribbed forms, and may appear in any part of the frond length. 

 This shows that new ribs may appear or old ones be suppressed at any 

 point in the growth of the plant. 



Variation in the midrib may affect the number of ribs in the follow- 

 ing three ways: (a) By its disappearance in the late growth of the plant 

 (Fig. 5) ; (b) By its appearance in a plant in which it was not present 

 (Fig. 3) ; (c) By its disappearance for a time and its reappearance farther 

 down (Fig, 4). Specimens were found showing all three of these varia- 

 tions in 4-5-ribbed combinations. The dropping of the midrib was found 

 in a 5-6 combination. At no point in a frond was a group of ribs found in 

 the middle ; thus there is no indication of the separation of the midrib 

 into several. Of course, in the stipe all the ribs arise very close together 

 and one might consider this the region in which the lateral ribs arise from 

 the midrib. 



