REPRODUCTION— THE FLOWER. 



63 



which they spring being known as the placenta. 

 Sometimes there is but one ovule, and this may 

 be at the base, side, or top of the ovary, and erect 

 or pendulous, straight or curved. Generally 

 the ovules are numerous, and if springing from 

 the edges of a single carpel, as in a pea, the 

 placenta is said to be sutural; if from the in- 

 folded dissepiments of a compound ovary, the 

 placenta is said to be axile, as in an Apple or 

 Lily; whilst if the ovules spring from the walls 

 of the cavity, the placenta is called parietal, as 

 in a Poppy. In the Primrose the ovules are 

 borne upon a little column which stands up in 

 the centre of the ovary detached from its walls. 

 This form of placenta is spoken of 2^ free central. 

 The ovule consists of a central mass, the 

 nucellus, surrounded by one or by two " coats " 

 which grow up over the nucellus from the base 

 upwards, being at first mere rings which lengthen 

 into tubes, and eventually cover the whole of 

 the nucellus with the exception of a small 

 opening at the top called the micropyle. When 

 the ovule is straight it is usually erect and sessile, 

 but when it is curved or inverted it is said to be 

 anatropal, in which case there is a stalk called 

 the raphe intervening between the base of the 

 ovule and the placenta, as may easily be seen in 

 a Pea or Hazel-nut. In the interior of the nucel- 

 lus is one large cell. called the embryo-sac, which 

 contains the egg-cell or germinal vesicle (fig. 66). 

 This, when fertilized by contact with the genera- 

 tive nucleus formed in the pollen-grain, develops 

 into the embryo plant. In addition to the egg- 

 cell the embryo-sac contains other cells and 

 nuclei, the nature and functions of which are 

 not fully ascertained. Mention may, however, 

 be made of the two polar nuclei. These are 

 at first free in the embryo-sac, but at or after 

 the period of fertilization the two polar nuclei 

 coalesce to form one cell, and from this cell is 

 ultimately formed the endosperm or albumen. 

 The other contents of the embryo-sac are per- 

 haps rudiments of an ancestral state, and may 

 eventually furnish links in the course of descent 

 from pre-existing forms, or points of similarity 

 with other forms as yet not made out. 



Fertilization. 



The essence of this process, the details of 

 which are very complex and still the subject of 

 inquiry, consists, as has been previously stated, 

 in the fusion of two previously separate masses 

 of protoplasm; the one, the male nucleus, acts 

 as the fertilizer and is derived from the pollen- 

 tube; the other, the female nucleus or oosphere, 



is formed in the embryo-sac. When fusion takes 

 place between the two nuclei the oosphere becomes 

 the oospore or egg, from which the embryo or 

 seedling plant is in due time developed. In 

 addition to the fusion of the two nuclei, it is 

 stated that a small portion of the protoplasm 

 from the pollen-tube fuses with a corresponding 

 portion of the protoplasm in the embryo-sac. 

 In any case we have a blending of two hereto- 

 fore separate things into one. 



The plant has thus the power of vegetative 

 propagation by means of spores or aggregations 

 of spores in the shape of buds, and of fertiliza- 

 tion by the fusion of male with female spores. 



Fig. 65.— Section of Style of Lilium Martagoii, showing Pollen-cells on 

 the Stigma, and sending down their Tubes along the conducting 

 tissue of the Style. 



Pollination is the name given to the transfer 

 of the pollen-grains from the anther to the 

 stigma. When deposited on the stigma, and 

 provided that organ be moist and in a receptive 

 condition, the pollen-cell commences to sprout, 

 and sends down a long slender tube through the 

 style into the ovary (fig. 65). This process 

 may occupy a long time, several months in the 

 case of conifers, and during that time the tube 

 not only grows but is nourished, partly on the 

 contents of the tube, partly on food obtained 

 from the style, a ferment being formed which 

 effects the solution of the food material and 

 enables it to be absorbed. The pollen -tube 

 forms from its protoplasm three nuclei: one is 

 called the vegetative nucleus, and has no part 

 in the process of fertilization, the other two are 

 smaller, and are known as the generative nuclei. 



Arrived in the ovary the pollen-tube comes 



