. Botany of the living plant. Botany. Fig. 214. Transverse section of the style of Rhododendron, show- ing the five-rayed channel lined T\'ith epithehum, and hlled with mucilage: it is traversed by the poUen-tnbes, which appear as compressed dots in the section. the growing tube. The nucleus of the vegetative cell with its cyto- plasm usually passes out first, while the aiitheridial-mother-cell is embedded in the rearward part of the vegetative cytoplasm. It soon divides to form two gametes, the nuclei of which follow the vegetative nucleus (Fig. 213, A). As the tube lengthens, the grain as we
. Botany of the living plant. Botany. Fig. 214. Transverse section of the style of Rhododendron, show- ing the five-rayed channel lined T\'ith epithehum, and hlled with mucilage: it is traversed by the poUen-tnbes, which appear as compressed dots in the section. the growing tube. The nucleus of the vegetative cell with its cyto- plasm usually passes out first, while the aiitheridial-mother-cell is embedded in the rearward part of the vegetative cytoplasm. It soon divides to form two gametes, the nuclei of which follow the vegetative nucleus (Fig. 213, A). As the tube lengthens, the grain as well as the older part of the tube is thus emptied of its con- tents. Successive lengths are then shut off from the distal part of the tube that is still full, by plugs of cellulose, so that as the tube advances it is still possible to preserve its turgor. Thus provided, the tube can advance through long distances to reach the ovule. (Compare Fig. 211.) Germinating on the surface of the stigma the negative aerotropism, positive hydrotropism, and positive chemotropism all lead the tube to a close relation with its moist tissues. Where there is an open channel the pollen-tube does not need to penetrate the tissue. Even where, as in Lily or Rhododendron, the channel is filledwithmucilage the tubes penetrate thesecretion,but not the cells which produce it (Fig. 214). There is little apparent difference in those cases where, as in Salvia, there is conducting tissue with mucilaginous walls (Fig. 205); for there the pollen- tubes penetrate the mucilagin- ous middle lamella, passing between the cells themselves. This is in fact the commonest way for the tube to enter the tissue of the stigma, and it is well illustrated in the Grasses. Here the tubes force their way between the stigmatic cells, penetrating their middle lamella. But occasionally the cells of the stigma are themselves perforated. This is seen in the Corn Cockle [Agrostemma), where the pollen-tube traverses the delicate cell
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1919
