58 MORPHOLOGV OF THE CELL. 



iTas attained a definite thickness, it becomes differenliated 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, /. 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 finally disappear. Lenticular grains {e.g. in the endosperm of 

 wheat) have a lenticular nucleus ; their layers grow most quickly in the direction of 

 the radii of a great circle concentric with it, and commonly split, the nucleus 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 nucleus becomes eccentric, is 

 further and further removed from the centre' of gravity of the grain, and is in 

 this case globular. In some ellipsoidal (in the cotyledons of peas and beans) or 

 elongated grains, the nucleus is extended in the direction of the longest axis. 



It is very common for two nuclei 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 nuclei 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 

 nuclei; 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 of 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 nuclei 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, 



