THE FOOD OF PLANTS. 17 



phyll bodies of Cladophora are already much like those of higher 

 plants. 



Mention may also be made here of a remarkable organism 

 which is met with in pools, clinging to the submerged parts of 

 plants. I refer to Hydra viridis, a small water animalcule, 5-12 

 mm. in length, and green in colour. If we transfer the Hydra to 

 a drop of water, and, without putting on a cover-glass for fear of 

 injuring the creature, examine it under the microscope, we shall 

 readily make out that it is a sac-like structure composed of two 

 layers, the ectoderm and the endoderm, that at the anterior end it 

 has a mouth opening (a posterior opening is wanting) surrounded 

 by many tentacles, and that it can contract itself and then again 

 stretch out. In the endoderm of the cylindrical body of the 

 Hydra, and also in the endoderm of the tentacles, we observe 

 numerous green globular structures. These are unicellular alge 

 which live in symbiotic relation with Hydra viridis. The Hydra 

 affords protection to the algas while they serve the Hydra by pro- 

 viding it, through their assimilatory activity, with organic sub- 

 stance and free Oxygen. 



In plant houses in which ferns are cultivated it is generally 

 easy to find on moist walls, or on the stems of tree ferns, fern 

 prothallia, small green mostly heart-shaped structures, closely 

 applied to the substratum. We remove some with the forceps, 

 and after rinsing them with water, examine them microscopically 

 in a drop of water. Except along the middle they are composed 

 of a single layer of cells ; they usually have a notch at their 

 anterior end, and produce on their lower or ventral surface fairly 

 long root hairs. For our present purpose it is specially im- 

 portant that in the green prothallium cells numerous chlorophyll 

 bodies are readily seen. 



It is further instructive to examine under the microscope a leaf 

 of the widely distributed moss Funaria hygrometrica. For reasons 

 which need not here be discussed, we select plants which have 

 been exposed for some time to diffuse daylight, and find that the 

 cells of the leaf, which up to the midrib is unilamellar, contain 

 many large chlorophyll grains, some of which are undergoing 

 division. 



We further prepare a transverse section through the thallus of 

 Marchantia polymorpha (a dichotomously branching liverwort, 

 very frequently occurring on moist soil). Without going into de- 

 tails it is sufficient here for us to determine the presence on the 



P.P. C 



