200 THE ORCHID REVIEW. (JULY, 1915. 
-and the pollen tubes effect an entrance by way of the tissues of the stigma, 
which is often elevated on a style. The great characteristic of the group is 
‘the high development of the flowers and fruit, the former depending upon 
the varying degrees of adaptation to fertilisation by insects, and the latter 
upon various devices for the dispersal of the seeds and their protection until 
the time for germination arrives. The female prothallium and the 
aarchegonium are now éliminated or no longer. recognisable, and the 
embryo-sac, after a few preliminary divisions, proceeds to the formation of 
the ovum, the development of the endosperm being postponed until after — 
fertilisation, while the stages in the pollinary processes are also simplified. 
The Phanerogamia now subdivide themselves into ‘two primary groups, 
Dicotyledons and Monocotyledons, the former characterised by having an 
embroyo with two cotyledons, a terminal growing point, and a permanent 
root system, the latter with the embryo reduced to a single cotyledon, and 
the growing point lateral, while the primary root: disappears early and Is 
replaced by adventitious roots, indicating a further degree of specialisation 
‘The further points of difference in the stem, leaf venation, and floral 
arrangement may be passed over. bs 
This brings us to the family Orchidacee, and we need not go into its 
-special characteristics, beyond pointing out the wide departure from normal 
methods in the development and fertilisation of the ovules. An Orchid 
flower-is only partially developed at the time of expansion, for the female 
generative system is'in abeyance, the ovules then appearing as minute 
points of undifferentiated tissue ; and they remain so until after the act of 
pollination, which applies the stimulus for their development. The first 
visible effect of pollination is the discoloration and withering of the perianth 
segments, and the thickening of the column, but while the pollen tubes are 
growing down the tissues of the latter the ovary and ovules are developing; 
and by the time they are mature the pollen tubes have reached their 
destination, and at once enter the micropyle of the ovule, and fertilisation 
is effected, after which the ovule develops into the seed. The interval 
between pollination and fertilisation may be a week or ten days in a hardy 
terrestrial: Orchid, up to about three ouaee: in oe Mossize, possibly 
longer in some cases. 
We thus see that Orchids, in which the Guosaeting point of develop- 
ment in the vegetable kingdom is reached, have a long and diversified 
ancestry, and a brief survey of its successive phases, with the progressive 
introduction of new and improved processes under changing conditions of 
the environment, forms a material help in the study of heredity. Without 
such a guide one might as well attempt the navigation of a rudderless ship 
across an uncharted ocean. Its application to the problems of ge 
tion and reversion must be left for a concluding chapter. R.A 
eS a ne 
