134 PLANT LIFE. 



formations; but apart from these doubtful examples it is 

 certain that algal remains occur as far back as the Upper 

 Silurian period, and in the older Carboniferous strata the 

 siliceous valves of diatoms have occurred, two species agree- 

 ing in the minutest details of structure with Epithemia gibba 

 and E. granulata, two species existing at the present day. 

 In the Cretaceous and later formations, various fossils or 

 impressions presenting affinities with the simpler green 

 algse and the Fucacese are met with. 



The amount of development and differentiation exhibited 

 by the Algse may be briefly summarized as follows : — 



(i) Vegetative Phase. — From the unicellular individual 

 a sequence of development through the colony-forming 

 species leads to multicellular forms, showing in many cases 

 a tendency towards the formation of special organs, as root, 

 stem and leaves ; but these structures are not of the same 

 morphological value, as in the higher members of the 

 Vegetable Kingdom. For example, the " root " of a seaweed 

 performs only the function of attaching the plant to a solid 

 body by a disc or branched rhizoids, and is not specialized 

 for the purpose of taking in food. Contemporaneous with 

 the gradual development of the thallus we find phases of 

 differentiation of its component cells, in the form of trumpet- 

 hyphse concerned with the transport of food, and the 

 shadowing in of tissue-systems. 



(2) Reproductive Phase.- — The most primitive method 

 of reproduction is purely vegetative, and consists in the 

 division of a unicellular individual into two daughter-cells, 

 both equally capable of growing into an individual re- 

 sembling the parent in form and size. This method, which 

 entailed the loss of the parent as an individual, was super- 

 seded by the differentiation of a small portion of the 



