328 Cleland. — The Cytology and Life-history of 
not approximate one to another closely enough at the periphery to form 
a membrane. 
It is the central region of the chromatophore, however, which is of 
greatest interest The appearance of this region varies according to the 
position of the cell of which it is a part. If the cell lies at the periphery of 
the thallus, where it is in the best position for assimilation, the central 
region consists of a spherical or ellipsoidal dark-staining area (Fig. 9), either 
minutely granular or quite homogeneous and amorphous-looking, in the 
centre of which is an intensely staining pyrenoid body, of homogeneous 
appearance. In a cell which lies towards the interior of the thallus, how- 
ever, we find quite a different condition (Fig. 11). Here the pyrenoid body 
is smaller and occasionally even altogether wanting. The homogeneous 
area surrounding it is replaced by a series of more or less granular threads 
radiating outward from the pyrenoid to the periphery of the central region. 
When the pyrenoid is entirely absent the threads occupy the entire central 
area. If we examine cells intermediate in position between the innermost 
and outermost (Fig. 10) we find all gradations between the two conditions 
just described. 
We must not conclude, however, that the conditions shown in Fig. 9 
will be gradually attained by the chromatophore in Fig. 11 as it grows 
older. It must be remembered that the growth of each assimilative fila- 
ment is limited, and this growth limit is reached a very short distance behind 
the growing apex. The result of this is that the cell shown in Fig. 9, which 
is the terminal cell of a mature filament, will remain the terminal cell of that 
filament (except in so far as it may cut off one or more trichomes) throughout 
the life of the plant. Furthermore, the cell shown in Fig. 11, which is 
situated somewhat below the periphery of the thallus, although it was at 
one time the terminal cell of the filament, has, at all times, held its present 
position with reference to the interior of the thallus — that is, close to the 
central core. In other words, this cell has not had the same assimilatory 
advantages that the cell in Fig. 9 has had throughout its whole life. There 
is, therefore, no direct transition from the condition in F'ig. 3 1 to that in 
Fig. 9. This is a natural result of the absence of intercalary growth. 
The method of development of these two conditions in the chromato- 
phore is illustrated in Fig. 8. Ch 10m atoph ores make their appearance 
wherever one of the active strands of the core region gives rise to a new 
assimilative tuft. If we follow the development of a single tuft through 
from the apex, we observe that a cell is cut off laterally from a central 
strand. At first this cell shows no signs of chromatophore, but very soon 
a structure looking like a small area of very dense cytoplasm is seen 
(F'ig. 8, a). This rapidly becomes denser and more distinct (Fig. 8, b). An 
aggregation of material then takes place towards the centre of the structure 
(F'ig. 8, c), where it becomes arranged into threads, the peripheral part 
