GENERAL SUCCESSION OF PLANTS IN TIME. 1479 



Thallophytes has been adopted. The divisions of the Fungi, as 

 being of no importance to palaeontologists in the present state of our 

 knowledge, have likewise been omitted ; as, also, have been several of 

 the groups of the Algae. In older botanical systems, plants, in place 

 of being ranged into the two primary divisions of Thallophytes and 

 Cormophytes, were separated into " Cryptogams " and " Phanero- 

 gams," the limits of these old divisions being indicated in the margin 

 of the above table ; and it will still be sometimes convenient to 

 employ the former term as a collective name for the non-flowering 

 plants. Similarly, the Cryptogams were formerly divided into Thal- 

 logens, Anogens, and Acrogens ; and we still very generally meet 

 with the two latter terms (the position of which is indicated in 

 brackets in the table) in geological works, where the expression 

 " Age of Acrogens " has become almost classical. It is, however, 

 very inadvisable to retain for geological purposes a classification 

 abandoned by the more advanced botanists ; and we shall therefore 

 adopt the names Bryophytes and Pteridophytes in place of the older 

 Anogens and Acrogens. 



The Thallophytes are cellular plants not distinctly differentiated 

 into stem and leaf; the Alga being distinguished from the Fungi 

 by the presence of chlorophyll. The Bryophytes, while still wholly 

 cellular, may have distinct stems and leaves ; while the Pteridophytes 

 are partly composed of cells and partly of long tubes or vessels, and 

 are thus fitted for the attainment of considerable dimensions. The 

 Phanerogams are like the Pteridophytes in being vascular plants, 

 but differ from all the preceding groups in developing flowers, which 

 in turn produce seeds containing an embryo. The series of the 

 Phanerogams is again divided into the Gymnosperms, with naked 

 seeds not enclosed in fruits, and the Angiosperms, in which true 

 fruits enclose the seeds. This latter group is further divisible into 

 two sections, known, from the number of the seed-leaves, as Mono- 

 cotyledons and Dicotyledons. In the Monocotyledons the embryo 

 has only a single seed-leaf or cotyledon, and the stems grow from 

 within and show no rings of growth ; the name of " Endogens " 

 applied to the group being based on this latter feature. In the 

 Dicotyledons, on the other hand, the embryo has two seed-leaves or 

 cotyledons ; and the group is sometimes spoken of as that of the 

 " Exogens," since the stems (as in the Gymnosperms also) grow from 

 without, with the formation of distinct rings of growth. 



General Succession of Plants in Time. 



As in the Animal Kingdom, so among plants there appears to be 

 a general correspondence between relative rank in the scale of organ- 

 isation and the order of appearance in time. Thus, as Sir William 



vol. 11. 2 p 



