J. BURTON ON THE REPRODUCTION OF MOSSES AND FERNS. 7 



the ferns, there is the equivalent of the spores and prothallus of 

 the Vascular Crytogams. 



Before concluding, I should like to say a few words on a general 

 principle suggested by my subject. It is a well-known fact that, 

 to ensure vigorous and healthy plants, an occasional return to 

 propagation by seeds is necessary. A constant recourse to vege- 

 tative reproduction by means of cuttings and similar methods 

 invariably results in degeneration and a liability to disease. 

 Sexual reproduction, therefore, evidently leads to a greater 

 development of vitality or strength of constitution than mere 

 vegetative reproduction. The principle I wish to bring to your 

 notice is another phase of this law. It is that the highest 

 structural development also is only attained by sexual reproduc- 

 tion as contrasted with that which occurs owing to the non- 

 sexual, even though both are included in one life cvcle. This is 

 very beautifully illustrated by the alternation of generations in 

 tin- two groups under consideration. In the ferns, for example, 

 the spores which are non-sexual produce a prothallus, a simple 

 cellular structure consisting mainly of a single layer of cells, 

 without vascular bundles, epidermis or true roots, showing alto- 

 gether but little differentiation of tissues. Upon it arise the 

 antheridia and archegonia. and on the union of their essential 

 parts — that is, on sexual reproduction taking place — a fern plant 

 results. It is very obvious that the structure of this is of a far 

 higher order than that of the prothallus. The fern has exceed- 

 ingly well-developed vascular bundles, epidermis and stomata — in 

 fact, all the structure of the highest plants; but from its non- 

 sexual spores, again, only the prothallus results. In the mosses 

 the case is not quite so evident, but a glance beneath the surface 

 shows that not less there also, a union of the sexes produces a far 

 higher development than the more simple process. The occur- 

 rence of the protonema tends to confuse comparison somewhat, 

 but it may be looked upon as only an early condition of the leafy 

 stem, and is not, in fact, a distinct organism. On examining, 

 then, the leafy stem arising from the non-sexual spore, it is found 

 to be of the simplest construction, notwithstanding its appearance 

 to the contrary. It is but a kind of parody of the outward 

 form merely of the vascular plants. It is all cellular tissue ; 

 there are no true vascular bundles, no epidermis, and, of 

 course, stomata are absent. The organs which I have called 

 " roots " are not roots morphologically, although acting as such. 

 We have seen here that the fertilised germ-cell gave rise to the 



