29 



CHAPTER III. 



Group I. 



THALLOPHYTA. 



This group includes the Algas, the Fungi, and the Lichens. 

 It embraces plants of widely different habit and complexity of 

 structure, both morphological and anatomical. In the lowest 

 forms thej^ are characterised by extreme simplicity in both 

 these respects, the plant body being sometimes a single cell, 

 sometimes a thallus consisting of filaments or plates of cells. 

 In the higher forms, on the contrary, the plants are often bulky 

 and formed of masses of tissue showing some considerable histo- 

 logical differentiation ; their form may display both root and 

 shoot, the latter exhibit - 



ing stem and leaves. Again • m j. 



these bulky masses may 

 be distinctly thalloid. 



The simplest Thallo- 

 phyte shows no histologi- 

 cal differentiation, being 

 only a single cell such as 

 Yeast, or Haematococcus. 

 A chain of cells like Nostoc 

 {fig. 779) is almost as 

 simple, though cells of 

 different appearance may be present in the chain. Usually 

 a filament of this kind has its cells independent, and sepa- 

 rated from each other by cell-walls. In a good man^^ cases 

 these separating walls are not formed, and the organism consists 

 of a tubular body with an external wall, on the inner face 

 of which lie the constituent cells, whose protoplasm is con- 

 tinuous throughout. The composite nature of this structure 

 is recognised b}^ the presence of numerous nuclei. A structure 

 like this is called a Coenocyte. 



The filament in other cases is much like this, but some of 



Filaments from a Nostoc colony. 

 After Luersseu. 



