242 THEORIES OF EMBRYOLOGY. 



with the ovule, are by some said to be derived from the pollen ; by 

 others, from the ovule itself; and by a third set, from the conducting 

 tissue of the style. 



503. Some maintain that the pollen-grains burst on the stigma, and 

 scatter the fovilla directly upon it, the influence of which is conveyed by 

 the conducting tissue to the ovule. Hartig seems to adopt this view 

 in some cases, as well as Gasparrini, who, from observations on the 

 Orange-tree and Cytinus, thinks that, as the result of this influence, 

 a filament or cellular prolongation is sent from the lower extremity of 



the style into the ovule. Almost all 

 modern physiologists discard this view, 

 and believe in the formation of pollen- 

 tubes, which were so ably demonstrated 

 by Brown in Orchidaceas and Ascle- 

 piadacea?, and have subsequently been 

 shown by Schleiden, Amici, Brongniart, 

 Meyen, Mohl, Mulder, Griffith, and 

 others. These tubes, which vary in 

 size, being often about 5 J U5 inch in 

 diameter, may be easily traced in many 

 instances, after the pollen has lain for a 

 few hours on the stigma ; for instance, 

 in Crocus, Salvia, Colocasia odora, Ges- 

 nera, (Enothera and Antirrhinum (fig. 

 434 bis). When an ovary, style, and 

 stigma are present, the tubes pass into 

 the conducting tissue, while in the case 

 of naked ovules, as in Conifer se and 

 Cycadaceae, the pollen comes into 

 434 bis direct contact with the foramen. 



504. The extent to which the tubes penetrate, and the mode in 

 which they give rise to the embryo, are matters of dispute. It is 

 maintained by some, that the tubes formed by the intine proceed only 

 to a certain extent down the style before rupturing to discharge the 

 fovilla, so that the influence of the latter is subsequently conveyed to 

 the ovule by the conducting tissue, and thus causes the formation of 

 an embryo. Mirbel and Spach, from observations made on Gramineaj, 

 as Zea Mais, and on the Yew and other Coniferae, have been led to 

 support this view. They believe that the tube does not reach the 

 ovule, that a primary utricle or vesicle (the first part of the embryo) 

 exists in the embryo-sac, or cavity of the nucleus, before fertilization, 



Fig. 434 bis. Portion of the stigma of Antirrhinum majus at the time of fecundation. ps,ps, 

 Superficial cells forming the papillse. tc, <c. Deep elongated cylindrical cells forming the con- 

 ducting tissue. </]), Grains of pollen attached to the surface of the stigma, the extine having 

 been ruptured, and the intine protruded in the form of tubes, tp, tp, which pierce the interstices 

 between the superficial stigmatic cells. 



