408 MORPHOLOGY. 



the growing extremity soon decays, e. g. in Nymphaa, Mirabilis, &c. 

 The differences stated render it wholly impossible to give safe indica- 

 tions a priori for all plants. Patience is necessary, not to be frightened 

 by frequent abortive attempts, till we have learned from the plant itself 

 its peculiarity : whoever has not this patience will not do for an inves- 

 tigator of nature. 



Many have referred to another difficulty in the observation of the 

 pollen-tube, which, from my own investigations, cannot at all be considered 

 as such, namely, the possible confusion of the cells with the conducting 

 tissue for the pollen-tubes. I have never yet met with a plant in which 

 such a mistake was possible; the cells of the conducting tissue are 

 always twice or three times as thick as the pollen-tubes of the same 

 plant : in no plant are those cells longer than very much elongated par- 

 enchyma-cells, i. e. about one-tenth of a line, and therefore every pollen- 

 tube may be recognised at once by the continuity of the cavity through 

 longer tracts. The lament over the possibility of that error has arisen 

 solely from very bad methods of investigation. He who takes an 

 impregnated plant, and hastily examines a longitudinal section from the 

 style, may perhaps doubt whether he has an elongated cell or a pollen - 

 tube before him ; but and this is the only proper way he who first 

 traces the development of the pistil in all its parts up to the time of 

 flowering, and then, well acquainted with the existing condition, 

 examines an impregnated pistil, perceives in a moment what new 

 elements have entered into the style, and cannot imagine the possibility 

 of a confusion of the pollen-tubes with the conducting tissue. Lastly, I 

 must confirm the opinion which Horkel has expressed, that Rob. Brown's 

 '* mucous tubes " are nothing else but the pollen-tubes, the connection of 

 which with the pollen-grain is already destroyed. Within a certain 

 time after impregnation all the pollen-tubes of the Orchidacece become 

 mucous tubes, because they begin to decay from without inward. 



Meyen says that he has frequently seen branched pollen-tubes : I have 

 never met with them, but I consider them very possible. Only in the 

 neighbourhood of the seed-bud, or quite inside the micropyle, I have 

 sometimes seen a very short blind lateral branch given off by a pollen- 

 tube, and, in general, the otherwise pretty smooth and cylindrical tubes 

 here very frequently exhibit irregular curves and varicosities. In the 

 earliest stage of formation of the tube, the contents of the pollen-tube 

 usually exhibit an active circulation, which, however, soon ceases ; by 

 degrees the contents becomes concentrated down into the apex of the 

 tube, partly unaltered, partly chemically changed into other matters, 

 often dissolved into a perfectly transparent watery fluid. 



It is well-known that the pollen-granules swell up and burst through 

 endosmose in water, and the coagulating contents are emitted in an 

 intestine -like form, but this has nothing to do with the formation of the 

 tube on the stigma ; on the other hand, true tubes may be obtained from 

 almost every kind of pollen, for clearer observation than is generally 

 possible on those taken from the stigma, by laying them in the sweet 

 juice which is secreted by some plants, e. g. in the nectary of the Crown- 

 imperial, the abundant nectar of Iloya carnosa, &c., or sometimes even 

 merely in sufficiently concentrated solution of sugar or diluted honey. 

 In these it usually is easy to see the circulation of the contents of the 

 pollen-cell in the formation of the tube, first observed by Amici. 

 Without human interference also, pollen-grains, which come accidentally 

 in contact with nectar, readily send out tubes, and we often find at the 



