590 PROGRESSIVE STAGES IN COMPLEXITY OF STRUCTURE. 



noticeably in the mai-ine Padina Pavonia, older species of which are composed of 

 superimposed, thin, leaf-like cell-plates. Systems of cell-masses are found in 

 many Florideiu and esjjecially in the large brown sea-wrack.s, Cystosira, Sargas- 

 suvi, and Fucus. The separate cell-complexes, which form a S3'steni in these 

 plants, frequently assume the form of leaves, and these sea-wracks are occasionally 

 ranked with leafy plants, which will be described later. Hydriirus and the 

 Stonewort (Ghara) are systems of cell-complexes; but while the individual com- 

 plexes in Hydrurus are connected very irregularly, they exliibit in Chara an 

 extremely regular, geometrical, whorled arrangement (c/. vol. II., fig. 20G '). 



Following this first division of systems, which have a uniform construction, is 

 the second, in which the body is built up of diflerent kinds of cell-complexes. 

 These are called mixed systems. Each member of such a mixed system, regarded 

 by itself, exhibits a simple homogeneous cell-tissue; but the simple complexes are 

 so combined that in one case cell-rows are carried by a cell-plate, while in another 

 case a cell-mass forms the starting-point for several open cell-nets, and so forth. 

 All possible combinations are realized in nature, but none more frequently than 

 that in which a cylindrical cell-mass forms the centre or axis of the whole plant- 

 body, whilst cell-plates or nets are laterally articulated. In many sorts of Bat- 

 rachospermuvi open nets are seen which are borne on a central pillar of cell-masses; 

 and in a liverwort (JungerTnannia trichophylla) the same thing occurs, except 

 that here there are cell-rows which proceed from the lower parts of the central 

 mass (fig. 140^). Many mosses and liverworts {e.g. Hoolceria splendens and 

 Jungermannia polyanthos) exhibit a stem-like central tissue which does not carry 

 cell-nets, but single-layered cell-plates. As shown in the illustration opposite, 

 all possible stages are to be seen in moss vegetation between central supports 

 provided with cell-nets, and those with cell-plates; this must be particularly noted 

 here, in order to establish the fact that all classifications and distinctions based 

 upon external forms are really only artificial, and that sharp limits between 

 the various forms do not exist. Still it conduces to clearness, none the less, if 

 we collect together and classify the various forms as well as we can. The mixed 

 systems which are represented by the liverworts, illustrated here, claim an 

 especial interest, inasmuch as they are to a certain extent the prototj'pes of plant- 

 bodies, i.e. of those complex forms which botanists in earlier times alone recognized 

 when speaking of the configuration of plants ; these alone were considered, for 

 example, in Goethe's Theory of Metamorphosis. 



The Plant-body is always membered, and each of its members is composed of 

 cell-complexes of the most varied kinds. In this lies the distinction between 

 plant-bodies and the previously-described forms. The members of a simple and 

 of a mixed system are simple cell-complexes: — cell-rows, cell-plates, and the like. 

 The members of a plant-body are, on the other hand, combinations of cell-rows, 

 cell-nets, cell-plates, and cell-masses. The cell-complexes combined in a member 

 of a plant-body are connected from their first origin. A single cell is always the 

 starting-point for the particular member of the body; this divides; the compart- 



