THE WONDERS OF LIFE 



tissues is more completely lost in proportion to the 

 closeness of their combination, the complexity of their 

 division of labor, and the differentiation and centraliza- 

 tion of the tissue-organism. Hence the various kinds 

 of tissue in the body of the histona behave like the va- 

 rious classes and professions in a state. The higher the 

 civilization and the more varied the classes of workers, 

 the more they are dependent on each other, and the 

 state is centralized. 



In the lower tissue-forming plants, the algae and fungi, 

 the plant-body has the appearance of a layer of cells, 

 the tissues of which show little or no division of labor. 

 In these thallophyta there are none of the conducting or 

 vascular fibres, the formation of which is of great im- 

 portance in the higher plants in connection with their 

 physiological function of circulation of the sap. These 

 more advanced vascular plants comprehend the two 

 great groups of ferns (pteridophyta) and flowering plants 

 {anthophyta, or phanerogams). Their body is always 

 composed of two chief organs, the axial stem and the 

 lateral leaves. This is also the case with the mosses 

 (bryophyta), which have no vascular fibres; they lie be- 

 tween the two chief groups of the non-vascular thal- 

 lophyta and the vascular cormophyta. However, this 

 histological and organological division of the two great 

 groups of tissue-plants must not be pressed; there are 

 many exceptions and intermediate forms. In general 

 their manifold tissue-forms may be brought under two 

 chief groups, which we may call primary and secondary. 

 The primary tissues are the phylogenetically older and 

 histologically simple "cell-tissues," such as we have in 

 the thallophyta (algae, fungi, and mosses) ; in these there 

 are no conducting fibres, or, at least, only rudimentary 

 ones. The secondary tissues are a later development 

 from these; they form conducting and vascular fibres 

 and other highly differentiated forms of tissue (cam- 



162 



