202 PROCEEDINGS OF THE AMERICAN ACADEMY 



in the thicker blades at this stage of growth. A series of sections at 

 and a little above the transition place will give all of the early stages. 

 The first indication that a cryptostoma is forming is the appearance of 

 a shallow saucer-shaped depression. This will be seen to be due to 

 the fact that the cells of the limiting layer in the region of this de- 

 pression cease to divide as actively in a tangential direction as they 

 have been doing, or as actively as their neighbors are doing. Further- 

 more, the cells of the outer cortex immediately below this point do 

 not increase in size either so rapidly or to so great an extent as their 

 neighbors do. So very soon this region is left behind by the more 

 active growth of the rest of the blade, and forms a slight depression. 



Very soon also the cells of the limiting layer at the bottom of this 

 depression begin to grow out into filaments, and a stage of this is 

 represented in P'igure 22. In this drawing the filaments or hairs are 

 only three or four cells in length, and still possess the original rounded 

 terminal cell. The growth is at the base, ;is may be seen from the 

 fierure, and as was described in the cases of the clusters of hairs found 

 on the primitive blade. The oldest cryptostomata of specimens of this 

 stage are decidedly bowl-shaped, but they lack entirely the prominent 

 overhangmg margin to be found m older specimens. 



Fourth Period. — Transitional Forms. — Having developed the 

 permanent holdfast, the plant enters the final period of changes. 

 Early in this, its active vegetative life ends, and it begins to mature 

 and to ripen. At the very beginning, it starts to throw off the old 

 blade, which has been, by means of its thinness and abundant crypto- 

 stomata, so well adapted for obtaining nourishment and for assimilat- 

 ino- it, and proceeds to develop a frond especially adapted for bearing 

 the reproductive bodies. 



Through an oversight, specimens of the stage showing the renewal 

 of the blade were not preserved otherwise than as pressed for the 

 herbarium, and so I am unable to give an account of the stage where 

 the transition takes place. But the change is brought about by 

 a thickeninc of the walls and a modification of the contents of the 

 cells, rather than by any change in the kinds of elements or in their 

 distril)ution. 



As mentioned above, the growth of the blade is very rapid at about 

 this time, and at the boundary of these two periods the plant attains 

 its maximum length. The most striking thing, perhaps, about one of 

 these large plants, such as were collected at Peak's Island and at 

 Naliant in the latter part of June, is the abundance and size of the 

 cryptostomata. It is in these plants that they reach their final com- 



