A Monograph of the GeniLS Alaria. i -r 



seraiblade extending horizontally from each margin. 



In young fronds, the midrib slants towards- the margins, 

 giving a biconvex shape to the cross sections. Some species keep 

 this character unchanged. But in others, the marginal parts 

 gradually thicken as the frond grows up, eventually to form 

 perpendicular edges. The cross sections are now compressed 

 rectangular. 



The thickness of the midrib varies according to the species as 

 well as to the age of the frond, often measuring 12 times as thick 

 as the laminai part. The breadth of the midrib in a blade is 

 nearly uniform from the base to the apex. In young post- 

 embryonal fronds, however, the upper part is naturally narrower 

 than the lower. 



The tissue elements and their arrangement in the midrib are 

 essentially equal to those of the complanated part of the stipe, 

 except that there are no mucilage glands in the former. The 

 greater part of the thickness is occupied by the cortical portion. 

 In other words, the elevation of the midrib above the blade is 

 mainly due to the remarkable development of the cortical tissue 

 (Plate XVIII, fig. 14). 



The structure of the cortical portion is practically the same 

 in the midrib and in the stipe. In the former, however, the cells 

 of the inner cortical portion are more thin- walled and greater in 

 diameter, uniformly cylindrical, and run closely parallel in longi- 

 tudinal direction. In the cross sections, these cells show angulate 

 polygonal shape, with narrow or scanty intercellular spaces, and 

 are disposed in compact, anticlinal rows (Plate XYIII, fig. 10). 

 In the blade, the cortical cells are much more loosely arranged and 

 take a decussate or patent course from the midrib as alluded to 

 above. 



