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 Bryophyta; 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 ALG, 
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 A/ge@ 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 Chondrus, 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 
B 
