1 68 Shiv Ram Kashyap. — The Structure and Development of 
The lower part of the prothallus, as stated above, consists of parenchy- 
matous cells without any air spaces between them. The cells are usually 
all similar, but sometimes some cells are elongated in the radial direction, 
and all are thin walled. All the cells in this part are full of starch grains, 
the chloroplasts which were present in the whole body in the early stages 
having been transformed into leucoplasts. This transition is gradual and the 
upper layers of the compact part are green like the lobes. 
The meristem is situated all along the margin (Figs. 16, 17) on the 
under surface, and it produces the parenchymatous cells of the compact 
Text-fig. 18. Text-fig. 19. 
*2 
Text-fig. 20. 
portion and the lobes of the upper spongy portion. The ventral position of 
the meristem explains the fact that the under surface of the prothallus is 
almost always concave. The upper spongy portion consists, as stated above, 
of lobes. Each lobe arises from the single projecting cell of the meristem, 
which divides at first by transverse walls and then by other walls in various 
directions. Later, some cells from the side of the lobe grow out and behave 
like the cells of the meristem, and the lobe thus becomes branched. The 
lobe at its base is several cells thick, but higher up it becomes thinner and 
the ultimate part is only a single layer of cells in thickness. The terminal 
cells in all cases are very turgid and either spherical or more or less deeply 
lobed and form a tuft (Figs. 23-27). 
