FERTILISATION. 81 



of Robert Brown that at the time of the expansion of an orchid 

 flower the ovules are only in a rudimentary state, consisting merely 

 of minute papilla? projecting from the pulpy surface of the placenta.* 

 The application of the pollen to the stigma must have a twofold 

 effect before the seeds can be perfected, first as a stimulant to 

 induce the maturity of the ovules, secondly to fertilise them by means 

 of the pollen tubes. It thence frequently happens, at least where 

 artificial means are employed, that the application of alien pollen, 

 the pollen of a different species and especially of a species belonging 

 to a different genus may bring about the first but fail to effect 

 the second, a circumstance that hybridists would always do well to 

 bear in mind. Under the artificial circumstances in which tropical 

 orchids are placed in the glass-houses of this country the period 

 for both processes extends over several months, which is known to 

 be much longer than is required in their native countries. 



The chief causes of this prolongation is the deficiency of direct 



sunlight, especially in the winter Avhen the sky is not only obscured 



by clouds often for several days in succession, but with the diminished 



altitude of the sun there is a corresponding diminution of intensity 



and potentiality in his rays. The capsules neither can nor do attain 



the perfection natural to them in their native countries, and it is more 



than probable that they yield but a fractional part of the quantity 



of good seed. 



In the absence of direct observation, the time required by species 



of epiphytal orchids to mature their capsules in their native home 



can only be approximately surmised from the times ascertained 



for the same species cultivated in the glass-houses of Europe. The 



earliest recorded observations of these times were made by Dr. 



Hildebrandt, in the Botanic Garden at Bonn, during the spring of 



1863, and the results of his observations were published in Mohl 



and Schlectendal's Botanische Zeitung, Nos. 44 and 45 of the same 



year. From these results we glean the following interesting facts : — 



The first species selected was Dendrobiam nohile, on account of its 



luunerous large flowers supplying favourable subjects for accurate 



observation. A large number of these were pollinated in the first 



and second weeks in January, and one of tliein was examined by 



making transverse and longitudinal sections of the ovary at intervals 



of every two days. Some flowers were fertilised with their own 



pollen, and others with tlie pollen of a different flower, but no 



* Observations on the Organs and Modes of J'^ecundation, \). Ij. 



