CHAPTER II. -PLANT TISSUES. 151 



antecedent living organisms. Such a thing as the spontaneous 

 generation of living protoplasm from inorganic matter has never 

 yet been observed. 



Plant Tissues. 



While it is true that all the essential phenomena which we 

 call "vital" are manifested within the compass of a single cell, 

 it is true, also, that the manifestation is feeble in comparison 

 with that exhibited by cell aggregates, where there is division of 

 labor among the cells. All the higher plants are such aggregates 

 or collections of cells. The Rose-bush, for example, is made up 

 of millions of them, and its life is not the mere aggregate life of 

 cells precisely alike, but rather that of sets of cells that have 

 grown to differ from each other in form and function, some 

 being specialized for one use, and others for another, but all 

 subserving the life of the whole organism. These cell-groups, 

 which differ from each other in ways more or less important, but 

 each of which is composed of similar cells, are called tissues. 

 The lowest plants cannot be said to possess tissues, since they 

 are either one-celled or are collections of precisely similar cells; 

 but as we study plants in the ascending scale, we find a more 

 and more complete differentiation of the cells, until, in the ferns 

 and flowering plants, we find a great variety of tissues, the indi- 

 vidual cells of which differ more or less markedly from the 

 typical cell already described. 



It must be remembered, however, that all these tissues origi- 

 nate from a single cell, and that each cell of the mature plant, 

 however great its deviation from the typical form, approximates 

 it very closely in the early stages of its development. 



Tissues may conveniently be classified into four series, the 

 parenchymatous, the prosenchymatous, the sieve, and the laticife- 

 rous series. 



I. — Parenchymatous Series. The tissues of this series all 

 agree in being less modified, in shape at least, from the primitive 

 or typical form of cells than the other tissues. They mostly 

 retain to maturity the proper character of cells ; that is, they 

 possess protoplasm and a nucleus, and possess the power of cell- 

 division. In some cases they become elongated and somewhat 

 fibrous in appearance, but more commonly they are not much 



