58 MORPHOLOGY OF THE CELL, 
has attained a definite thickness, it becomes differentiated by further growth into 
three layers. If it is a dense layer, watery substance becomes intercalated in its 
middle, and the dense layer splits into two layers separated by a less dense one. 
But when a watery layer becomes sufficiently thick, its middle lamella may be- 
come denser, and a new dense layer be thus formed between two less dense 
layers. This process of splitting of the layers depends on their increase in thick- 
ness ; and since this itself is the most vigorous where the layers are intersected 
by the longer branch of the axis of growth, the splittings, i. e. the new formations of 
layers, take place there most abundantly, and least so on the opposite side of the 
nucleus, where they may even entirely cease. The layers of the more quickly 
growing side of the grain become, as they pass to the slowly growing side, 
gradually thinner, and finall^V disappear. Lenticular grains {e.g. in the endosperm of 
wheat) have a lenticular nuctouc ; their layers grow most quickly in the direction of 
the radii of a great circle concentric with it, and commonly split, the hucTcuc re- 
maining central. If, on the other hand, the growth takes place in one direction 
(e. g. in the ovoid grains of the potato-tuber) the nuolouc^ becomes eccentric, is 
further and further removed from the centre of gravity of the grain, and is in 
this case globular. ^ some ellipsoidal (in the cotyledons of peas and beans) or 
elongated grains, the micleuc is e^ctended in the direction of the longest axis. 
It is very common for two nuoloi to form in a small young grain ; round each 
of them layers are formed, and the growth is strongest in the line of union. The 
distance of the nuofoi from one another becomes continually greater ; thus a tension 
arises in the few layers which are common to both ; this leads to the formation 
of^ an inner fissure, which lies at right angles to the line of union of the two 
wii^cT ; it is continued towards the outside, and the grain breaks up into two 
half-grains which may nevertheless adhere to one another. If this division occurs 
more often, compound grains arise, consisting of numerous secondary grains, the 
number of which may amount even to thousands {e.g. in the endosperm oi Spinacia 
and Avena). 
Compound grains of from two to ten secondary grains, with a mulberry-like 
appearance, are extremely common in the parenchyma of quickly growing plants, 
e. g. in seedlings of Phaseolus and stem of Cucurbita. Grains of this description 
are different in their origin from compound grains of the kind which occur in 
chlorophyll; in this latter case a number of small grains exist from the first, 
which only touch and adhere to one another in consequence of increase of size. 
(See Fig. 45, p. 47.) 
Partially compound starch-grains result when new wuoloi with their surround- 
ing masses of layers are formed after the grain has already formed several layers. 
The secondary grains appear therefore to be inclosed within the layers of the 
mother-grain. In this case also tension arises from the unequal growth of the 
common layers and of those belonging to each secondary grain, leading at 
length to the formation of fissures ; but these do not usually extend to the outside ; 
the secondary grains remain united. 
(a) The growth of starch-grains by intussusception must be inferred from the fol- 
lowing considerations:— Supposing that the formation of layers occurs by deposition, 
