WEALDEN PLANTS. 



Group THALLOPHYTA. 



A heterogeneous group of plants, many of which consist of a 

 single cell, or of a thallus without any external differentiation 

 into stem and leaf structures. Some of the higher members have, 

 on the other hand, well-defined external and internal differen- 

 tiation. The group is distinguished from the higher plants by 

 the absence of true roots and vascular (conducting) tissue. In 

 this respect it agrees with the Bryopliyta ; but in the latter the 

 division of the vegetative body into stem and leaf structures is 

 more pronounced, and the method of sexual reproduction and 

 alternation of generations in the life-cycle are characters which 

 separate the two groups. 



Class ALGJE. 



Plants provided with chlorophyll, and therefore capable of de- 

 composing carbonic acid gas, assimilating the carbon and giving 

 off oxygen, under the influence of sunlight. The vegetative body 

 occurs in all forms, from that of a single cell to a complicated 

 multicellular structure, made up of assimilating, conducting, pro- 

 tective, and other tissues. 



The custom of naming the innumerable markings or impressions 

 which constitute many of the so-called fossil Alga by the same 

 generic titles as are applied to recent seaweeds is to be strongly 

 deprecated. Several observers have furnished ample proof of the 

 exceedingly small value to be attached to the determinations of 

 " algal" impressions, and have thus given a wholesome check to 

 the indiscriminate naming of these most doubtful fossils. The 

 application of such terms as Caulerpites, Chondrites, etc., to 

 markings on stone which have a greater or less resemblance to 

 the recent genera Caulerpa, Chondria or Chondrm, is very likely 

 to lead us astray, and to be especially misleading to those who 

 are apt to draw conclusions as to the past history of living genera 

 from the occurrence of their names in fossil lists. As an instance 



