OF ARTS AND SCIENCES. 201 



ture of the midrib agrees completely with that of the stipe. A com- 

 parison of the two with the adult shows that the midrib undergoes 

 the less chauge of structure, remaining always more primitive in con- 

 dition. The lamina of such a irond as we are considering consists 

 of two or three rows of large, squarish, thin-walled cells, covered by 

 epidermal layers of much smaller cubical cells, containing the brown 

 pigment. (Fig- 3.) 



About simultaneously with the appearance of medullary threads in 

 the stipe, they are seen between the large-celled layers at the base 

 of the lamina, and gradually push out towards its edge as the plant 

 grows older. The lamina does not further increase in complication of 

 structure for a considerable time. 



4. Perforations of the Lamina. 



The very young lamina of Agarum Tiu-neri is quite imperforate for 

 a time, but when a length of 3 to 4 cm. is attained, the perforations, 

 afterward so characteristic, begin to be formed in all parts of the 

 frond. They continue to be developed during the life of the plant, 

 especially at the base of the lamina and near its middle along the 

 midrib, but also wherever growth has produced a considerable un- 

 perforated space. Any large frond will furnish an abundance of 

 perforations in all stages of development. (Figs. 4 and 5.) The 

 rarity of the occurrence of such holes among plants and the evident 

 definiteness of their development give to their study much interest 

 and importance. 



The first indication of the formation of a hole is the appearance, on 

 either of the faces of the lamina, of a small, conical, papilla-like ele- 

 vation, with a corresponding depression on the opposite surface. 

 (Fig. 5, a.) This papilla continues to push up until a rupture occurs 

 at its tip (Fig. 5, b), and a small opening, like a pin-hole, is made 

 through the lamina (Fig. 5, c). If a piece of a lamina with papillae 

 in various stages of development be held to the liglit, the tips of the 

 papilliE will be seen to be translucent, and those most so which are 

 nearest the rupturing point. This shows that there is a gradual de- 

 crease in the thickness of the tissue at the tip of a papilla, from the 

 time of its beginning until the rupture takes place. The hole now 

 increases in size with the growth of the frond (Fig. 5, d), and may 

 reach 2 cm. in greatest diameter and become very irregular in shape, 

 though usually of a broadly elliptical form. It retains for a long time 

 the character of a perforation of the lamina with a raised margin 



