SECTION 17.] 



BRY0PHYTE3. 



165 



and very hygrometric threads (called Elaters) whicli are thought to aid in 

 the dispersiou of the spores. (Fig. 542-544.) 



502. Marchantia, the commonest and largest of the true Liverworts, 

 forms large green plates or fronds on damp and sliady ground, and sends up 

 from some part of the upper face a stout stalk, ending in a several-lobed 

 umbrella-shaped body, under the lobes of which hang several thin-waUed 

 spore-cases, which burst open and discharge spores and elaters. Riccia 

 nafans (Fig. 545) consists of wedge-shaped or heart-shaped fronds, which 

 float free in pools of still water. The under face bears copious rootlets ; in 

 the substance of the upper face are the spore-cases, their pointed tips 



merely projecting : there they burst open, and discharge their spores. 

 These are comparatively few and large, and are in fours ; so they are very 

 like the macrospores of Pillworts or Quillworts. 



503. Thallophyta, or Thallophjrtes in English form. This is the name 

 for the lower class of Cellular Cryptogams, — plants in which there is no 

 marked distinction into root, stem, and leaves. Roots in any proper sense 

 they never have, as organs for absorbing, although some of the larger 

 Seaweeds (such as the Sea Colander, Fig. 553) have them as iioldfasts. 

 Instead of axis and foliage, there is a stratum of frond, in such plants 

 commonly called a Thallus (by a strained use of a Greek and Latin word 

 which means a green shoot or bough), which may have any kind of form, 

 leaf-like, stem-like, branchy, extended to a flat plate, or gathered into a 

 sphere, or drawn out into threads, or reduced to a single row of cells, or 

 even reduced to single cells. Indeed, Thallopliytes are so multifarious, so 

 numerous in kinds, so protean in their stages and transformations, so re- 

 condite in their fructification, and many so microscopic in size, either of 



Fig. 542. Fructification of a Jungermannia, magnified; its cellular spore-stalk, 

 surrounded at base by some of the leaves, at summit the 4-valved spore-case open- 

 ing, discharging spores and elaters. 543. Two elaters and some spores from the 

 same, highly magnified. 



FiQ. 544. One of the frondose Liverworts, Steetzia, otherwise like a Junger- 

 mannia ; the spore-case not yet protruded from its sheath. 



