1388 



POLLEN 



tlieu, is to be found ii i-pason for the bpnutv anil special- 

 ization of external wall. In entomopliilous poUiii the 

 elliptical form of grain predominates, but the general 

 shape is extremely various; and the plants proilucing 

 such pollen are usually provided with beauty of flower, 

 fragrance, or other insect attraction. 



In order that the pollen which has been trans- 

 ported to the stifjma may be effective, it must be 

 healthy. Experiments have shown that weak, poorly 

 nourished orchard trees often proiluce 

 iiietrective pollen. The nature of the 

 season may also have great influence 

 upon its character, continued rains 



causing great losses by preventing the maturity of this 

 product as well as by mechanical injury and by pre- 

 cluding the winged carriers. Most plants have some 

 special provision for the protection of the pollen against 

 rain; that is, either by the closing of the flower under 

 moist conditions, or by the location of the anthers in a 

 sheltered tube, under projecting hairs, lobes, or other 

 corolla appendages. 



The individual particles of pollen are in the form of 

 delicate grains only readily visible in some quantity, as 

 in powdery taasses. At the time when they are set free, 

 the grains are generally entirely distinct from one 

 another, to be blown about by an accidental wind or 

 carried by visiting insects. In some cases, however, 

 the grains are bound together loosely or by means of 

 delicate glutinous threads (Rhododendron); they may 

 be closely united into4's (heath family); or the whole 

 tissue of an anther or its divisions may remain intact as 

 pollinia (some orchids, milk-weed, etc.). A particular 

 species of plant will produce pollen quite constant in 

 form and attire; but an aggregation of cultivated varie- 

 ties originated from a single species may show consider- 

 able variation in this regard. Nevertheless, form, size, 

 color, surface markings, texture of wall, and trans- 

 luoency of contents are not fixed qualities even for re- 

 lated genera or species. See Figs. 1870-3 for different 

 forms of pollen. 



When the healthy pollen of one plant falls upon the 

 ripe stigma of a plant of the same species, the giains 

 germinate in the sugary excretion of the stigma by the 

 protrusion of a tube which penetrates the style and 

 effects fertilization as described under Fertifisittion. 

 Furthermore, it is well known that while the flowers 

 of many plants may be readily fertilized by their 

 own pollen, the offspring ai-e stronger when pollen 

 from another plant or another variety have had access 

 to the flower. Sometimes pollen from a foreign variety 

 is absolutely essential to the best fruit formation. This 

 is particularly true of certain varieties of the pear. A 

 poor quality of fruit can be prevented only by growing 

 together different varieties. Again, although a plant 

 may readily pollinate itself, yet the pollen from another 

 plant or variety maybe prepotent over its own. That is 

 to say, if the plant be pollinated by its own pollen along 

 with that of a foreign varietv, that of the foreign vari- 

 ety will usually effect fertilization. This can be ex- 

 plained only on physiological grounds, and at present 

 merely from a theoretical point of view. Any pollen 

 penetrates and effects fertilization because it is at- 

 tracted, first by substances in the style, and later by 

 the egg-cell itself. When a foreign variety is prepotent 

 It is so because it is more readily attracted, due, we may 



POLLINATION 



say, to a greater difference of potential between the 

 two elements, the two elements from the sanje plant 

 being more in equilibrium and less markedly attractive. 

 As regards pollen from a foreign species, it seems to be 

 the rule that hybridization does not occur so readily, 

 and we must then assume that the differences have be- 

 come so great as to cause repulsion. 



The detailed development of pollen is highly interest- 

 ing and instructive on morphological grounds, but in 

 this place a very brief account of the formation of the 

 grains will sufllce. The developmental phases in Big- 

 nonia venusta will serve as an example. A cross-section 

 of the young flower-bud will show that in the anther-sac 

 regions, semicircular layers of large well -nourished 

 cells ( called archesporial cells ) are differentiated. 

 These cells divide and the layer increases in extent, 

 yet in this case it is always only one cell in thickness. 

 \Yhen these cells have finally attained considerable size 

 and provided themselves with a thick wall, they divide 

 more or less simultaneously; and then each of these 

 daughter-cells divides again by a division following 

 quickly upon the first. Each cell has then formed four 

 new cells within its original walls. The new cells re- 

 iiiKin thus united in 4's until each is provided with a 

 stout vA\ of its own, and then they separate. Each cell 

 is then an immature pollen - grain, and technically a 

 spore, that is, exactly homologous with the microspores 

 of the vascular cryptogams. As a rule, before these 

 pollen-grains are set free, another change occurs de- 

 noting maturity. This consists in the division of the 

 nucleus of the spore in such a way that two cells of 

 unequal size result (in some conifers several small cells 

 are formed). On germination the large cell, which now 

 incloses the smaller, protrudes the tube which pene- 

 trates the style; whereas the nucleus of the small cell 

 divides into two, and one of these fuses with the egg- 

 cell in the ovule, thus fertilizing it. 



Allied topics are discussed under PoUinutioii, Self- 

 sterilifi/, Flower, and Hybrids. g. jj, DudOAR. 



POLLINATION. In botanical usage, Pollination is 

 the transfer of pollen from the anther to the stigma. In 

 horticultural usage, particularly with reference to ondiard 

 fruits, the term is often a]>plied in a general way to desig- 

 nate all the influenrrs concerned in the setting of fruit. 

 Aside from rliosc- i-ases in which the stamens and pistils 

 are so intimati-ly a-^-^o.-iated that the pollen falls directly 

 on the stigma. Il.iwi is are pollinated mainly in two ways: 

 by wind and by insects. The grasses, sedges and pines 

 atford examples of wind-pollinated plants. The flowers 

 of wind-pollinated plants are u.sua!ly inconspicuous 

 and without nectar or fragrance. They produce a great 



1873. Pollen erains of Abutilon striatum (above). 



Bedding geranium (Pelarganium hortorum. on right). 



Chrysanthemum (on lower left). All magnified. 



abundance of light, dry pollen, which is wafted away by 

 the slightest breeze and is often carried many miles by 

 a strong wind. The pistils of these plants are usually 

 long and feathery, and thus are well adapted to catch 

 the flying pollen. For the benefit of those who are un- 

 informed in botany, it may be said that pollination is 

 concerned primarily with the "essential organs" of the 



