148 DIFFERENCE OF ORGANS AT DEVELOPMENT STAGES 



form is an inherited character ; how far we are justified in regarding it as 

 an original one, or as acquired through adaptation, will be discussed when 

 I speak of the Bryophyta in the second part of this book, and therefore 

 I only briefly refer to the group in this place. I shall subsequently show 

 that external circumstances also have been proved in a number of cases 



FIG. qo. Bryum pseudotriquetrum ? Cushion of protonema upon a piece of wood. The conditions for growth 

 having been most favourable this unusually large development of protonema has taken place, and at the same time 

 the production of moss-buds has been retarded. Natural size. 



to retard the appearance of the adult form, and therefore the duration of 

 the juvenile form can be prolonged beyond its usual period (Fig. 90). The 

 peculiar behaviour of many conifers in this respect will be noticed presently. 

 I now proceed to give some illustrative instances from different groups 

 of the Plant Kingdom : 



I. THALLOPHYTA. 



In Oedogonium, Vaucheria, Fucus, and some other Algae, the sexually-mature 

 plants grow from the spores without any essential changes of form, but there are other 

 Algae in which this is not the case and they produce usually a more or less peculiar 

 pro-embryo. In illustration of this some Florideae may be mentioned, amongst which 

 two freshwater forms, Lemanea and Batrachospermum, have been carefully investigated. 



Lemanea has a pluricellular cylindrical thallus, which must be regarded as built up 

 out of cell-threads which have fused together, and it produces the sexual organs. The 

 spores produced by these organs grow out into a much simpler ' pro-embryo ' ' composed 

 of cell-threads and upon it the sexual shoots, which have a more complex structure, 

 appear. In this plant as in other instances, mosses, for example (alike those with 

 filiform protonema and those with flat pro-embryo as in Sphagnum), the adult form 

 possesses characteristic root-threads the configuration of which conforms with that of 

 the pro-embryo, and from them new plants can shoot out. They spread upon the 

 substratum and fix the thallus to it. Similar pro-embryos may also, as Brand has 

 recently shown 2 , be produced by the vegetative cells of the sexual shoots if these live 



1 This was first shown by Thwaites, 'On the early stages of development of Lemanea fluviatilis,' 

 in Proc. of the Linnean Society of London, vol. i (Feb. 15, 1848), p. 360. See also Wartmann, 

 quoted by Goebel in Flora, 1889. 



2 Brand, Fortpflanzung und Regeneration von Lemanea fluviatilis, in Ber. d. deutsch. bot. Ges., 

 xiv. p. 185. 



