PLANTS WITH A DISTINCT AXIS AND FOLIAGE. 71 



consist of a single layer of cells. Most frondose Sea-weeds, 

 as well as Lichens, Liverworts, &c., are made up of several 

 such layers. This is not the place to illustrate the almost end- 

 less diversity of forms under which the frond, or, as it is called 

 in Lichens and Fungi, the Thallus^ appears in these lower grades 

 of plants ; nor to notice their particular modes of propagation ; 

 except to say, in general, that the spores are still nothing but 

 specialized cells, developed in some one of the ways already 

 explained. But we now begin to meet with special organs or 

 peculiar apparatus in which the reproductive cells are formed, 

 instead of occurring indifferently in any part. 



104. Plants of a Tissue of Cells combined into a solid Axis, or with 

 stem and branches. Stem-like solid forms occur, perhaps as abun- 

 dantly as the leaf-like or frondose, in the higher representatives of 

 the lowest orders of plants, in Algse, Fungi, and Lichens ; and oc- 

 casionally the two are somewhat vaguely presented in the same 

 individual. Thus, many of the larger Sea-weeds display a leaf-like 

 frond on the summit of a solid stalk ; this stem, however, has once 

 formed a part of the leaf. But in the Liverwort Family the dis- 

 tinction is first clearly exhibited, and in the true Mosses the higher 

 type of vegetation is fully realized, namely in 



105. Plants with a Distinct Axis and Foliage; that is, with a stem 

 which shoots upward from the soil, or whatever it is fixed to, or 

 creeps on its surface ; which grows onward from its apex, and is 

 symmetrically clothed with distinct leaves as it advances. All 



FIG. 84. Fruit-stalk, with a portion of the foliage, of a Jungermannia, magnified, to show 

 its entire cellular structure. 



FIG. 85. One of the tubular spirally-marked cells from the fruit of a Jungermannia (a) ; and 

 (6) the spiral threads which result from its disruption. Some of the spores stick to the tube. 



FIG. 86. Jungermannia Lyellii, less than the natural size. 



