70 THE ORCHID REVIEW. [Marcu, 1909. 
and consist of a minute speck of protoplasm, containing a nucleus, chloro- 
phyll (the green colouring matter by which nutrition is carried on), and 
cell sap, the whole being surrounded by a cell wall. Within the limits of 
this cell wall the whole life of the plant is carried on. Propagation is 
effected by division: the nucleus divides, each half secretes a new cell wall, 
and the cycle is complete. Progressing upwards we find forms in which 
the cells have begun to cohere in rows, and others in tissues, representing 
the first stage of co-operation between individual cells, and soon the stage. 
is reached where individual cells, or aggregations of cells, are set apart for 
a special purpose, entailing differentiation of tissues within the organism. 
Here we find the first indications of the sexual process. Two cells, 
externally alike, though obviously differently organised, unite, and their 
contents coalesce, forming a zygote, which afterwards develops into a new 
individual, like the one from which it was derived. A greater differentia- 
tion of the conjugating or sexual cells follows, and the process has since 
passed through increasing degrees of specialisation, ultimately reaching the 
degree of complexity seen in the higher plants to-day. The original asexual 
method of propagation is still retained by some of the simplest plants. 
The earliest plants were aquatics (Algz), but the gradual adoption of a 
terrestrial habit led to a new stage of development, to meet the new con- 
ditions of existence. In the mosses this took the form of the production of 
what is known as the moss fruit, the production of a mass of minute 
asexual spores, contained within a capsule. On reaching maturity the 
spores escape, become disseminated, and on alighting on a damp place 
germinate directly, without any sexual process, into,the moss plant, thus 
introducing a marked alternation of generations. This moss fruit repre- 
sents a new or spore-bearing stage, called the sporophyte, the spores being 
minute, cheaply-produced reproductive cells, which are capable of inde- 
pendent growth without fertilisation, and can rest uninjured while the 
conditions remain unfavourable. In the ferns the sporophyte stage 1s 
carried much further, the sexual stage (or gametophyte) being reduced to a 
small cellular prothallus, which bears (in special receptacles) the sexual 
cells, the female after fertilisation developing into the fern plant (or 
sporophyte). This latter has a large branched vascular system, on which 
the asexual spores are borne in enormous numbers. These on germinating 
reproduce the prothalloid stage or gametophyte, thus completing the cycle. 
In the flowering plants the gametophytic stage is still further reduced, and 
has no longer an independent existence, being contained within the body of 
the sporophyte. Fertilisation is effected there, after which further develop- 
ment takes place, a seed is formed and ripened, after which, with certain of 
its envelopes, it separates from the parent, giving rise to a new and highly 
complex generation, the spermatophyte. 
