SECT. II PHYSIOLOGY 279 



Peronosporeae, certain Ascomycetes). In both cases the separating 

 wall is ultimately absorbed and the contents of the male cell passes 

 into the female cell and fuses with the latter. 



In the most highly developed plants, the Phanerogams, the male 

 cell, which is here usually reduced to a nucleus, is also brought into 

 relation to the egg-cell by a process of growth. 



The egg-cell is within the ovule, and this is enclosed by the ovary, 

 and the sperm-cell is formed in the pollen-grain. The pollen-grain 

 has to be brought to the tip of the carpel (stigma) ; this process is 

 known as pollination. There it forms a tubular outgrowth, the pollen- 

 tube, which grows down the whole length of the style, enters the ovule, 

 and ultimately liberates the sperm-cells at its tip. In some 

 Phanerogams the pollen-grains and the ovules are formed in distinct 

 male and female flowers ; these may either be borne on the one plant 

 or on distinct individuals. In any case a transference of the pollen to 

 the stigma is necessary, and this is never effected actively by the 

 plant, but by external means of transport. 



It will be seen in the description of the Phanerogams that currents of air and 

 water, as well as certain animals which frequent flowers, are employed for the 

 purpose of effecting pollination. 



AYhere the male and female organs are present in the same 

 flower, i.e. in hermaphrodite flowers, it might be assumed that 

 no special arrangements would be necessary to bring the pollen to 

 the stigma. More accurate investigation has, however, shown that 

 such adaptations exist in abundance and are often of the most detailed 

 nature. They do not, it is true, aim directly at the conveyance of the 

 pollen to the stigma of the same flower ; often they render such 

 SELF-POLLINATION impossible, and effect CROSS-POLLINATION, i.e. the 

 conveyance of pollen to the stigma of another flower on the same 

 plant or on another individual. 



The product of fusion of the egg and sperm-cell surrounds itself, 

 as a rule, with a cell-wall. In the lower plants an oospore or 

 zygospore is thus formed which germinates, usually after first under- 

 going a period of rest. In the higher plants growth and cell-division 

 take place forthwith ; an embryo is produced which in Bryophyta and 

 Pteriflophyta continues its further development, while in the Phan- 

 erogams it soon enters on a period of rest. Before thi.s, however, a 

 number of stimuli have proceeded from the development of the 

 embryo ; these are especially complex in the Angiosperms. The 

 ovule in which the embryo is enclosed commences to grow ; it enlarges 

 and assumes a characteristic structure. It has developed into the seed, 

 and this as a rule is liberated from the ovary and, after a resting 

 period, germinates. The ovary also grows actively after fertilisation 

 and develops into the fruit The variety in fruits cannot be entered 

 upon in this place. 



