. Physiological botany; I. Outlines of the histology of phænogamous plants. II. Vegetable physiology. Plant physiology; Plant anatomy. DESCENT OF THE POLLEN-TUBE. 431 pistil with a large number of ovules the stigmatic surface is large, as is also the amount of conductive tissue of the stj-ie through which the pollen-tubes are to descend. 1118. The conductive tissue through which the pollen-tube descends, and by which it is nourished, is formed at the stigma by a modification of epidermal ceUs, and below this arises from modifications in the parenchyma; in the style it may constitute a solid ma
. Physiological botany; I. Outlines of the histology of phænogamous plants. II. Vegetable physiology. Plant physiology; Plant anatomy. DESCENT OF THE POLLEN-TUBE. 431 pistil with a large number of ovules the stigmatic surface is large, as is also the amount of conductive tissue of the stj-ie through which the pollen-tubes are to descend. 1118. The conductive tissue through which the pollen-tube descends, and by which it is nourished, is formed at the stigma by a modification of epidermal ceUs, and below this arises from modifications in the parenchyma; in the style it may constitute a solid mass of delicate cells, sometimes with walls which have undergone the mucilaginous modification, or it may simply line the hollow tube which is frequently found, as in the pistil of the violet. 1119. The time required for the descent of the pollen-tube de- pends upon the length and character of the path the tube is to traverse, and is verj- different in different cases. Hofmeister states that in Croons vernus, with a style which is from one to two inches in length or sometimes more, the tube reaches the ovary in from one to three days. Schleiden ^ gives the following times required for descent of the tube: Cereus grandiflorus, having a style nine inches long, a few hours; Colchicum autnmnale, with a stj'le thirteen inches long, twelve hours. In some other cases (certain orchids) it is weeks before the end of the tube lias descended for even a very short distance. 1120. A single pollen-grain of some flowers can emit more than one pollen-tube : thus Amici has seen twent}' to thirty' tubes proceed from one grain. Pollen-tubes sometimes branch in their course downward. 1121. The length of tii^ae during which pollen-grains can preserve their vitality has been determined for a few cases : ^ 1 Schleiden: Principles of Scientific Botany, 1849, p. 407. 2 Gartner, quoted by Mohl: Vegetable Cell, p. 134. Fig 196. Diagram of a longitudinal section of an nvary liaving only one ovule with bas
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