PAET II. 

 TISSUES AND SIMPLE ORGANS. 



A. STRUCTURE OF TISSUES AND SIMPLE ORGANS. 



The anatomist distinguishes between formed tissue, we permanent 

 tissue, and tissue in the process of formation, or formative tissue. 

 Formative tissue tissue which is capable of growth and cell-division 

 is in general designated as meristem. Again, a distinction is made 

 between short-membered parenchymatous formative tissue, or meri- 

 stem in the narrower sense, and longitudinally extended formative 

 tissue (more prosenchymatous in nature), or cambium. It is readily 

 understood that every cambial tissue is more or less secondary in 

 nature, for in general every organ begins with short or spherical 

 cells (" primary rneristem "). 



By an organ is understood a cell portion, a cell, or a cell-complex, 

 adapted for a definite function. Tissue is a purely morphological 

 conception. Any coherent cell-complex having extension in at 

 least two directions may be designated as " tissue." The considera- 

 tion of the structure of organs and tissues necessarily coincides with 

 the discussion of the structure of the plant itself, since plants are 

 either single cells i cell-threads, cell-surfaces, or cell-bodies* 



The building up or the formation of the three plant-forms last 

 named depends on the one hand upon the mode of cell-division, and 

 on the other upon the growth of the cells, individually and in mass. 



A cell-thread or cell-filament is a cell-complex whose septa are 

 at right angles to the longitudinal axis, or which, at least, presents 

 no longitudinal septum when revolved upon its longitudinal axis. 

 (Numerous examples may be found among the algae and fungi, as 

 well as among various trichomes of higher plants.) 



45 



