THE CAUSE OF UNIONS. 87 



unions to take place between tlie parts of any whorl, form- 

 ing "cohesions," and also between different whorls, or 

 "adhesions." 



Other causes may determine them, for hypertrophy may 

 set in through a purely vegetative stimulus ; for it is not 

 unfrequent to see abnormal cohesions and adhesions in cul- 

 tivated orchids, such as petals or sepals adhering to the 

 column, etc. Such may, with a good deal of probability, 

 be referred to the artificially stimulated conditions under 

 which they are grown. These abnormal cohesions between 

 members of the perianth, and adhesions to the column, have 

 been observed both in this country and America.* As a 

 particular instance of the latter kind, Mr. Meehan had 

 observed several dozens of flowers of Phaius grandiflorus which 

 had the dorsal sepal united to the column, all being confined 

 to separate spikes from those which have perfect flowers. 

 In some cases, of the same plant two of the petals were 

 united so as to form a hood over the column. 



Another peculiarity of Orchids is the tendency to convert 

 sepals or petals into labella, and to multiply the spurs when 

 an orchid is characterized by them so as to render them 

 peloric, a sure sign of hypertrophy. f 



All these "monstrosities" seem to point to an excessively 

 unstable condition of equilibrium in the flowers of Orchids ; 

 and that they are peculiarly sensitive to the effects of nutri- 

 tive stimuli, whether brought about by visits of insects or 

 by artificial cultivation. So that the order Orchidece is 

 particularly interesting, as furnishing indirect or even direct 



* As by Mr. T. Meelian. Proc. Acad. Nat. Soc. Phil, 1873, pp. 

 205, 276. 



t The remarkable influence of the presence of a "plant-bug," 

 causing the normally irregular corolla of Clerodendron to become 

 hypertrophied and peloric, will be described hereafter (p. 130). 



