FORMATION OF CELLS. ' 7 



cell-wall, contract their protoplasmic body, together with all its serviceable contents 

 (starch, oil, chlorophyll, &c.), expel the water of the cell-sap, and form a new cell. 



We may pass over the innumerable intermediate forms, and turn cur attention at 

 once to the other extreme, namely those plants of which each usually consists of thousands 

 or even millions of cells, as is the case with Vascular Cryptogams and Phanerogams • 

 and in which at the same time the different parts of the plant undergo an entirely 

 different morphological development, and are adapted to different functions for the 

 support of the v.hole. Here then we find that certain cells never attain their full 

 development, they remain . constantly in the condition of youth which is represented 

 in Fig. I, A ; these however assist the whole by continually giving rise to new 

 cells by division, which then, on their part, undergo a further development. Such 

 cells, which serve exclusively for the purpose of producing new ones, are found at the 

 extremities of all roots and branches, and abundantly at the base of leaves. The cells 

 produced in these positions undergo a different development according to their situa- 

 tion, and usually in such a way that whole aggregations of them into layers or strings 

 follow the same course of development. Some grow quickly in all directions, their 

 wall remains thin, the great bulk of their protoplasm becomes transformed into 

 chlorophyll, they are rich in cell-sap, and serve, as we shall see hereafter, for as- 

 similation, /. e. the production of new organic substance, which is formed out of the 

 elements of the absorbed nutritive material ; in other parts of the same plants the 

 cells extend greatly in length, their diameter remains small, they form no chlorophyll ; 

 a certain number remain succulent and serve to conduct certain assimilated sub- 

 stances ; other cells of the Same string thicken their walls rapidly in many ways, 

 their septa become absorbed, numerous cells in the same row join into a long tube 

 (vessel), from which the protoplasm and the cell-sap disappear ; they serve then as 

 air-passages for the interior of the plant. In their neighbourhood are formed the 

 wood-cells ; they are mostly fibre-like, extended in length, their wall greatly thickened, 

 and its substance chemically changed (lignified) ; they form collectively a firm frame- 

 work which supports the remaining tissues, lends firmness and elasticiLy to the whole, 

 and is especially adapted for the rapid conduction of water through the tissues of 

 the plant. In the tissue of tubers, bulbs, and seeds, most of the cells remain thin- 

 walled ; they become filled in the interior with albuminous substances, starch, fat, 

 inuline, &c., which afterwards, when new organs are being formed, serve as material 

 for the construction of new cells. In the same manner a considerable series of other 

 forms of tissue could be adduced, cork, the testa of seeds, the stone of stone-fruit, &c., 

 which all alike attain the needful firmness and strength by a peculiar development 

 of their cell-walls, in order to serve as protective envelopes for the other masses of 

 cells which are still capable of further development ; their contents disappear as soon 

 as the cell-wall has assumed these properties, and their purpose has been fulfilled. 



Each of the forms of cell hitherto spoken of, occurring in one and the same plant, 

 thus serves especially or exclusively for one purpose only ; in correlation with this, 

 either the cell-wall, the protoplasmic body, the chlorophyll, the cell-sap, or its granular 

 deposits, is specially developed. Very commonly these specialised cells lose the power 

 of reproduction and of multiplying by division ; when they have fulfilled their function, 

 they disappear, or their woody frame-work, the cell-wall, alone remains. The whole 

 plant, of which these cells form a part, continues to remain as such ; at definite places it 

 possesses cells, which, at the proper time, again produce new masses of cells, and these 

 again are adapted to fulfil for the time all these functions. 



Sect. 3. Formation of Cells ^ — The formation of a new cell always 

 comnjences with the re-arrangement of a protoplasmic body around a new centre ; 



* H. von Mohl, Vermischte Schriften botanischen Inhalts, Tubingen 1845, pp. 6t., 84, 362.— 

 Schleiden in Miillei's Archiv, 1838, p. 137.— Unger, Botan. Zeitung, 1844, p. 489 ; H. v. Mohl, 



