5o MORPHOLOGY OF THE CELL. 



the direction of most vigorous growth, which is continuous in a straight or curved 

 line with the direction of least vigorous growth ; this line is called the axis of the 

 grain ; it always passes through the nucleus. 



The growth of the grains of starch is accomplished exclusively by intussusception ; 

 new particles of the formative material become intercalated between those already 

 existing both in a radial and tangential direction, by which means the proportion 

 of w^ater at particular places is at the same time changed. The youngest visible 

 globular grains of starch consist of denser less watery substance ; in this is formed 

 subsequently the central watery nucleus; in the latter a central part may become 

 denser, and in this, when the increase in size has advanced sufficiently, a softer 

 nucleus may again arise. It may however also happen, after a softer nucleus has 

 arisen surrounded by a dense layer by differentiation of the original dense nucleus, 

 that in the dense layer a new soft one may arise, and thus become split into two 

 dense layers, the inner of which encloses the soft nucleus. The layers increase 

 by deposition in thickness and circumference. When a layer has attained a definite 

 thickness, it becomes differentiated by further growth into three layers. If it 

 is a dense layer, watery substance becomes deposited in its middle, and 

 there arises in the dense layer, which now splits into two lamellse, a less dense 

 layer. But when a watery layer becomes sufficiendy thick, its middle lamella may 

 become denser, and a new dense layer is formed between two lamellae of a less 

 dense one. This process of splitting of the layers depends on their increase in 

 thickness ; 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, ensue there most abundantly, least often on the opposite side of the nucleus, 

 and may even entirely cease there. The layers of the more quickly growing side 

 of the grain become, from bending round on the slowly growing side, constantly 

 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 here most commonly split, 

 the nucleus remaining 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 common layers which surround both; this leads to the forma- 

 tion 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, perfectly 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). 



Perfectly compound grains of from two to ten half-grains, with a mulberry-like 



