12 



LECTURE VIII. 



by means of subsequent growth, we observe between each two chambers (as Fig. no 

 shows) not a double wall, but three layers. The cavity of each one of the 

 neighbouring cell-chambers is in fact enclosed by a more or less thick layer; 

 while between the two lies a middle layer, which usually behaves differently 

 towards chemical reagents. This is either capable of swelling to a great extent, or 

 is very strongly lignified or cuticularised. I distinguished this layer fifteen years 

 ago, in order to exclude every theoretical expla- 

 nation as to its origin, as the middle lamella of 

 the thickened partition-wall of adjoining cells. 



As one of the most general results of the his- 

 tological researches of the last four or five decades, 

 it is remarkable that the whole internal structure 

 even of the most highly organised plants arises 

 from cells in the manner described. The older 

 phytotomists were by no means of opinion that 

 all the histological elements of highly organised plants 

 could be considered as cells : they distinguished, 

 rather, three chief forms, viz. cell-tissue, fibres, 

 and vessels. Fibres are the long-drawn elements 

 of the bast and wood running to a point above 

 and below ; vessels, again, are long prismatic or cylin- 

 drical tubes which run particularly in the wood. 

 It was one of Mohl's greatest services to demon- 

 strate that the fibres, as well as the vessels and 

 all other elements of the plant, arise by the pro- 

 gressive growth and changes of form of primitive 

 ordinary cells. For as, according to what has 

 been already said, the leaves, roots, and such 

 organs are constructed in the form of outgrowths 

 from the growing-points of the higher plants; so 

 also all the various forms of tissues arise from 

 the embryonal-tissue of the growing-point. In this 

 we find, in fact, very small thin-walled cells, all 

 alike, and entirely filled with protoplasm and cell- 

 nucleus : these cells are constantly multiplied during 

 their growth by the intercalation of new partition- 

 walls. The youngest organs which grow forth 

 from the growing-points themselves consist of such 

 embryonal tissue; and in proportion as the young 

 organs increase externally in size and complete 

 their configuration, the embryonal tissue in the interior also undergoes further 

 changes. The tissue, quite homogeneous at first, becomes differentiated into layers 

 and strings of various nature; and with the growth of the organ itself, this 

 internal differentiation into various forms and systems of tissues proceeds also, 

 until in the fully grown organs the typical tissue forms have attained com- 

 plete de\'elopment. It is not my object to describe in detail this progressi\'e 



FIG. 



of thickened 



cells, showing the middle lamella (m); i the 

 whole mass of cellulose lying upon this middle 

 lamella; / lumen of the cell from which the 

 contents have been removed. A from the cor- 

 tical tissue of the stem of Lycopodium cliama- 

 cypariisHs ; B wood cells from the inner wood of 

 a young fibro-vascular bundle of Helianthus 

 ajtnuus; C wood of Piiius sylvestris, st a 

 medullary ray. 



