ORIGIN OF CONDUCTING TISSUES. 173 



With regard to tbe difficulty of rearing Orchids, the 

 reader may be referred to the Report on the Orchids Confer- 

 ence,* ia which Mr. B. T. Lowne observes : " One of the 

 difficulties in rearing seedling Orchids arisLS, I believe, from 

 the fact that the pollen is only developed from the prolifica- 

 tion of the mother cells, after the pollinia are placed on the 

 stigma." He also found that, besides the pistil thus stimu- 

 lating the pollen, "the stimulation due to the presence of 

 the pollinia gives rise to the development of the capsule, 

 even whilst the ovules remain unimpregnated." f 



The significance of the above details lies in the fact that 

 external influences, both mechanical and physiological, can 

 bring about changes in the epidermal X and sub-epidermal 

 layers, with a determination of a flow of fluids of a specific 

 character to those specialized tissues. As this is proved to 

 be true for the conducting tissues, so do I infer it to be 

 equally so for glands of various kinds. 



* Journ. of Roy. Hart. Soc, vol. vii. ; see paper by Mr. H. J. Veitch, 

 p. 22. 



t L.c, p. 48. " Degeneracy " will be discussed in Chaps. XXVI. and 

 XXVII. 



X M. Mer found that stomata were developed in the epidermis of 

 galls on vine-leaves which normally had none. "Insolation" or 

 exposure to light has a marked influence on the orm of the epidermal 

 cells, and in increasing the number of stomata. The walls become 

 straighter and thicker, and especially the cuticle. M. Mer believes the 

 production of stomata to be the direct result of the accumulation of 

 nutrient substances. Comp. Rend, xcv., 1882, p. 395. See also Journ. 

 Roij. Mia: Soc, 1882, p. 530, and 1883, p. 91. See above, p. 154. 

 Another important paper on the same subject, fully corroborating 

 these observations, has lately appeai-ed, by M. L. Dufour, entitled, 

 Influence de la Lumiere sur la Forme et la Structure des Feuilles, 

 Ann. des Sci. Nat., 7 ser., torn. 5 (1887), p. 311. 



