94 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



structure almost as neat as in vessels with compound spires ; 

 though, sometimes, the principal part of the endochrome is 

 simply collected into a single irregular band-like thread. The 

 vascular tissue of plants in general, as observed before, is a mere 

 modification of cellular tissue. All tissue, in fact, in an infant 

 state, is cellular, and cell-walls, apparently simple, consist some- 

 times, if not always, of spirally-arranged fibres. These are visible 

 enough ui many Phaenogams. In Fungi we have the same 

 structure, in Podaxon (Fig. 6, c), and Agardh has shown that 

 the cell-walls of some Algae, as Conferva melcigonium (as 

 stated above, p. 8), have a spiral structure. The elder Agardh 

 recognised the analogy between the woody fibres of plants and 

 the threads of Conferva, and the analogy in poiat of structure 

 is borne out as well as in more palpable attributes. We have 

 already seen that the stems of the larger Algae exhibit a structure 

 somewhat simdar to that of Exogens, but the resemblance does 

 not stop here : in such genera as Mesoglcea there is a distinct 

 horizontal as well as vertical system ; and what is very curious, 

 in Batrachospermum and some species of Callithamnion 

 (Fig. 26), the thickness of the stem is increased externally by 

 a descending system from the branches. In such Algae, this 

 is no merely theoretical notion, but one which a single glance 

 at the microscope will confirm ; and though the theory of Du 

 Petit Thenars is now exploded as explanatory of the growth of 

 Exogens, no one will deny that their growth is in direct pro- 

 portion to the number of healthy buds which are developed. 

 Kutzing has figured dot-Kke canals (tiipfel of the Grermans), 

 in Laminaria digitata, and I have seen something of the 

 same kind, though not so distinctly as in his figure, in a vertical 

 section ot Lessoniafuscescens. 



73. In the simplest Algae there is no distinction of root, stem, 

 or leaves. Some, in fact, consist of nothing more than cells, 

 either entirely free, or floating in a common gelatine, and are 

 multiplied by a constant division of the endochrome. Where, 

 however, reproduction takes place by means of a spore, as in 

 Isthmia, there is very generally something, however slight, of 

 the nature of a stem, though the produce of this spore may, 

 for many generations, be propagated only by division ; and, in 



