ELEMENTARY OEGANS. 119 



are thus distinguislied. from proper vessels by tlieir transparent walls and by 

 branching. 



TTnion of the Elementary Organs. — Botanists are divided in opinion as to the forces 

 which cause the walls of the elementary organs to cohere ; some think that the 

 walls of the cells are originally semi-fluid, and hence become agglutinated, and 

 remain so even after the plant has ceased to live ; others consider that an inter- 

 cellular secretion cements the adjacent cell-walls. A third opinion is that vegetable 

 tissue originates as a homogeneous plasma, which gradually thickens, and ends by 

 forming vacuoles, which afterwards become the cavities of the cells ; a common 

 septum therefore separates the neighbouring cells; but soon each cell becomes 

 individualized, the septum doubles more or less completely, and the cohesion between 

 the cells is due to an interposed cellular tissue. This theory differs from the 

 second, inasmuch that in the latter the cells are cemented by a subsequently secreted 

 matter, while in the former the cells are united by an unorganized tissue, developed 

 cotemporaneously with themselves ; this unorganized tissue then itself becomes 

 cellular, and finally separates the previously individualized cells which' it originally 

 united. Communication is established between elementary organs in various ways ; 

 it has been stated that it takes place by means of the destruction of the contiguous 

 surfaces of cells and fibres placed end to end, from which there results a vessel ; 

 communication can also be established through the walls of cells, either by the 

 disappearance of the outer membrane, or by slits or holes at different points of its 

 wall, or simply by pores rendering these membranes permeable. 



Contents of the Elementary Organs. — The contents of these are very various: 

 gaseous, liquid, or solid. Cell-contents appear as scattered or agglomerated 

 granules, which in very young cells usually assume a lenticular form, and rest 

 against the wall, or are even buried in 'its thickness (fig. 664) ; this body (the nucleus, 

 cytohlast, or phacocyst of the cell) is regarded by botanists as a germ which, by its 

 development, will produce a new cell. In most cases the nucleus becomes less 

 distinct as the cell develops. According to the recent labours of M. Hartig, the 

 nucleus is principally formed of small particles of matter analogous to albumen, a 

 certain number of which are transformed into vesicles, which again give origin to 

 cellulose, fecula, chlorophyll and aleurone. Cellulose is an insoluble substance forming 

 the cell- walls, fibres, and vessels, the composition of which is. identical in all 

 plants. Woody tissue or lignine is nothing but the thickened and condensed cellu- 

 lose ; to its density wood owes its hardness ; the stony particles in 

 the flesh of pears and the stones of fruits are also formed of it. 



Fecula or starch may be recognized by its blue-violet tinge when 

 acted on by iodine, by its insolubility in cold water and its coagu- 

 lation in hot water ; its chemical composition is that of cellulose. ' 

 Starch-grains are generally spheroidal or irregularly ovoid (fig. 671) ; 

 their surface presents concentric circles around a point which usually 

 occupies one of the ends of the granule. These circles indicate so 

 mauy layers, superimposed around a small nucleus ; thus the 

 starch-grain is developed from within outwards, that is, in the reverse way to the cell 



