52 VEGETABLE HISTOLOGY. 



CHAPTER IX. 



TISSUES OF VARIOUSLY MODIFIED CELLS AS FOUND IN HIGHER 



PLANTS. 



While it is true that all the essential phenomena which we 

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

 is also true that the manifestation is feeble in comparison with 

 that exhibited by cell aggregates, where there is division of 

 labor among cells. All the higher plants are such aggregates 

 of cells. They are made up of millions of them, and their life 

 is not the mere aggregate life of cells precisely alike, but rather 

 that of sets of cells that have come to differ from each other in 

 form and function, 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. There is a great variety of tissues, the 

 individual cells of which differ more or less markedly from the 

 typical cell already described. 



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

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

 however great its deviation from the typical form, approximates 

 the latter very closely in its early stages of development. 



The various kinds of tissues are classified into four groups 

 or series. 



1. Parenchyma; ordinary soft ground 

 tissue. 



I. Parenchymatous 

 Series. 



2. Collenchyma; thick-angled tissue. 



3. Sclerotic parenchyma; stony tissue. 



4. Epidermal tissue. 



5. Endodermal tissue. 



6. Suberous or corky tissue. 



II. Wood or libriform tissue. 

 2. Tracheids or vasiform cells. 

 3. Ducts or vascular tissue. 

 4. Hard bast or bast-fibres. 



III. Sieve Series, including only sieve or cribriform tissue. 



IV. Laticiferous Series, including laticiferous or milk- 

 tissue, which may be simple or complex. 



Not all of the tissues are alive at maturity. Some are dead 

 and merely serve as supporting tissues. All of the prosen- 

 chymatous series are of this nature, being devoid of protoplasm 

 and mechanical in function. Likewise sclerotic and suberous 

 tissues. Any one of the higher plants will contain most of the 

 above tissues. If we examine stems of plants we find that the 

 tissues are not arranged indiscriminately, but always follow a 

 definite order. This order in Phanerogams is different from 

 that in Cryptogams, in exogenous flowering plants it is dif- 

 ferent from that in endogenous ones. But in any particular 



