FIRST GROUP. 



THE THALLOPHYTES. 



UNDER this name are included the Algae, Fungi and Lichens, whose vegetative 

 body usually consists of a thallus, and shows no differentiation into stem, leaf and 

 root, or such differentiation is only rudimentary; at the same time if we set out from 

 the simplest forms which show no outward distinction of parts, we find in different 

 divisions of the Thallophytes indications of advance towards a higher differentiation ; 

 and in the most highly developed representatives of the separate divisions the outward 

 distinction of parts goes so far, that the conceptions of stem and leaf are as applicable 

 in their case as in that of higher plants ; a true root, such as that of vascular plants, 

 is however always wanting in the Thallophytes, though certain organs are usually 

 present which in a physiological and functional sense we may designate as roots ; but 

 they are always distinguished from the roots of vascular plants by the absence of 

 a root-cap and by their branching not being endogenous. 



The internal differentiation of the Thallophytes starts like the external from 

 stages of the utmost conceivable simplicity, and ascends through numberless transi- 

 tions to more and more complex forms of cell and tissue ; still in the most highly 

 developed forms we nowhere meet with that differentiation into sharply separated 

 systems of tissue, which we are able to discriminate in the higher plants as dermal, 

 fundamental 1 and fascicular ; the homogeneity of the tissue is a striking fact even 

 where the thallus consists of extensive masses of tissue, as in large Fungi. A 

 noteworthy peculiarity of some large Algae (Laminarieae) is that their stems, like 

 those of Gymnosperms and Angiosperms, are capable of a secondary growth in 

 thickness due to the presence of a peripheral zone of meristem. 



The Thallophytes, notwithstanding the comparative simplicity of their organisation, 

 afford a great variety of examples of the manner in which the process of development 

 starting from the simplest organic forms passes by very different ways to forms which 

 are more and more highly differentiated and more and more perfect internally and 

 externally. The whole vegetative body in its simplest stage consists of a single small 

 cell with a thin, smooth cell-wall, and cell-contents in which protoplasm, chlorophyll, 



1 Sachs in his Lehrbuch, 4th ed., p. 121, gives the name of fundamental tissue to the masses of 

 tissue in the higher plants, which remain over after the first formation and further development 

 of the dermal tissue and the vascular bundles. 



B 2 



