298 PHOTOSYNTHETIC SYSTEM 



plastic material. According to Stahl, indeed, when Marchantia poly- 

 morpha is grown in sunlight, its photosynthetic filaments actually 

 come to resemble a palisade-tissue ; in these circumstances, namely, the 

 individual cells of the filaments are elongated in palisade fashion, that 

 is to say with their long axes at right angles to the surface of the 

 thallus. 



In foliose Liverworts, and in the majority of Mosses, the photo- 

 synthetic system has remained at a low level of organisation ; usually 

 it comprises only a single layer of cells, and serves simultaneously as 

 the efferent tissue. The individual cells are accordingly often elon- 

 gated so as to form curved series running obliquely from the apex to 

 the base of the leaf and inwards towards the midrib ; in this case the 

 removal of synthetic products is effected in part at least by the midrib. 

 In Polytrichum the photosynthetic tissue takes the form of longitudinal 

 plates or lamellae, which stand up perpendicularly from the upper side 

 of the leaf ; each plate consists of a single layer of cells, and is separated 

 from its neighbours by fairly wide air-spaces. In its essential features, 

 therefore, the photosynthetic system of Polytrichum agrees with that of 

 Scmpervivum. In the genera Aloina and Crossidium the broad midrib 

 bears, in the apical half of the leaf, a number of erect dichotomously 

 branching cell-filaments, containing abundant chlorophyll. In the 

 former genus, the wings of the leaf consist of a single layer of cells 

 poorly provided with chlorophyll, and arch over the photosynthetic 

 filaments after the manner of a protective epidermis ; a cross section of 

 the leaf recalls a side-view of one of the air-chambers of Marchantia. 



A far more elaborate photosynthetic system is found in many Moss- 

 capsules, 153 where it is located partly in the wall of the sporogonium 

 proper, and partly in the so-called apophysis. Even in the exaggerated 

 form which it assumes in the Splachnaceae, the apophysis represents 

 nothing more than the distal end of the seta, which has become 

 specialised for photosynthetic activity, just like an Equisetum-stem or a 

 twig of Spartium [or other switch-plant]. According to the author's own 

 observations, the wall of the capsule of Fnnaria hygromctrica (Fig. 124) 

 contains two layers of photosynthetic elements. The outer of these is 

 largely composed of typical funnel-cells. The inner layer, on the other 

 hand, consists of elements resembling spongy parenchyma-cells. It is 

 connected to the outer spore-sac by cellular filaments (trabeculae), which 

 serve to convey the synthetic products manufactured in the wall to the 

 developing archesporium across the intervening air-space. The photo- 

 synthetic system of the massive apophysis consists of typical palisade- 

 tissue, and forms a three- to five-layered sheath around the clavate 

 parenchymatous core which represents a prolongation of the vascular 

 bundle of the seta ; just above the core, a little central mass of spongy 



