2734 



POLLEN 



POLLEN 



The pollen of the pine has even developed bladders, 

 so as to be borne more lightly upon the wind. On the 

 other hand, those plants largely dependent upon the 

 visits of insects for pollination may have the pollen- 

 grains provided with some kind of spines, ridges, fur- 

 rows, or viscid coatings that they may the more readily 

 adhere to hairy limbs or other surfaces of the insect 

 which may come in contact with them. Here, then, is 

 to be found a reason for the beauty and specializa- 

 tion of external wall. In entomophilous pollen the 

 elliptical form of grain predominates, but the general 

 shape is extremely various; and the plants producing 

 such pollen are usually provided with beauty of flower, 

 fragrance, or other insect attraction. 



In order that the pollen which has been transported 

 to the stigma may be effective, it must be healthy. 

 Experiments have shown that weak and poorly nour- 

 ished orchard trees often produce ineffective pollen. 

 The nature of the season may also have much influence 

 upon its character, continued rains causing great 

 losses by preventing the maturity of this product as 

 well as by mechanical injury and by precluding the 

 winged carriers. Most plants have some special pro- 

 vision for the protection of the pollen against rain; that 

 is, either by the closing of the flower under moist con- 

 ditions, 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 masses. At 

 the time when they are 

 set free, the grains are 

 generally entirely dis- 

 tinct from one another, 

 to be blown about by 

 an accidental wind or 

 carried by visiting in- 

 sects. In some cases, 

 however, the grains are 

 jjjj~ bound together loosely 

 ^ or by means of deli- 

 cate glutinous threads 

 (Rhododendron); they 

 may be closely united 

 in fours (heath family) ; 

 or the whole tissue of 

 an anther or its divi- 

 sions may remain intact 

 as ppllinia (some orchids, milkweed, and others). A 

 particular species of plant will produce pollen quite 

 constant in form and attire; but an aggregation of cul- 

 tivated varieties originated from a single species may 

 show considerable variation in this regard. Neverthe- 

 less, form, size, color, surface markings, texture of wall, 

 and translucency of contents are not fixed qualities 

 even for related genera or species. See Figs. 3094- 

 3097 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 grams 

 germinate in the sugary excretion of the stigma by the 

 protrusion of a tube which penetrates the style and 

 effects fertilization as described under Fertilization 

 (Vol. Ill, page 1221). Furthermore, it is well known 

 that while the flowers of many plants may be readily 

 fertilized by their own pollen, the offspring are stronger 

 when pollen from another plant or another variety 

 has 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 may be prepo- 

 tent over its own. That is to say, if the plant be pol- 

 linated by its own pollen along with that of a foreign 



3097. Pollen-grains of Abutilon 

 stria turn (above). Bedding gera- 

 nium (Pelargonium hortorum, on 

 right). Chrysanthemum (on lower 

 left). (All magnified) 



variety, that of the foreign variety will usually effect 

 fertilization. This can be explained only on physiologi- 

 cal grounds, and at present merely from a theoretical 

 point of view. Any pollen penetrates and effects fertili- 

 zation because it is attracted, 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 say, to a greater difference of 

 potential between the two elements, the two elements 

 from the same plant being more in equilibrium and 

 less markedly attractive. As regards pollen from a 

 foreign species, it seems to be the rule that hybridiza- 

 tion does not occur so readily, and we must then assume 

 that the differences have become so great as to cause 

 repulsion. 



The detailed development of pollen is highly inter- 

 esting and instructive on morphological grounds, but in 

 this place a very brief account of the formation of the 

 grains will suffice. The developmental phases in Big- 

 nonia (Pyrostegia) 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 differ- 

 entiated. These cells divide and the layer increases in 

 extent, yet in this case it is always only one cell in 

 thickness. When these cells have finally attained con- 

 siderable 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 remain thus united in fours until each is pro- 

 vided with a stout wall 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 denoting 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 penetrates 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. 



B. M. DUGGAK. 

 Pollination. 



In botanical usage, pollination is the transfer of pollen 

 from the anther to the stigma. In horticultural usage, 

 particularly with reference to orchard fruits, the term 

 is often applied in a general way to designate all the 

 influences concerned in the setting of fruit. For the 

 benefit of those who are uninformed in botany it may 

 be said that pollination is concerned primarily with 

 the "essential organs" of the flower, the stamens and 

 pistils. The stamens bear the pollen in their anthers, 

 and they die after the pollen is shed. The pistils bear 

 the ovary or seed-case, the style, and the stigma. The 

 pollen falls upon the stigma. In some plants these 

 organs are separated in different flowers or even on 

 different plants. (Fig. 3098.) 



Aside from those cases in which the stamens and 

 pistils are so intimately associated that the pollen falls 

 directly upon the stigma, flowers are pollinated mainly 

 in two ways: by wind and by insects. The grasses, 

 sedges, and pines are examples of wind-pollinated plants. 

 The flowers of wind-pollinated plants are usually incon- 

 spicuous and without nectar or fragrance. They pro- 

 duce a great 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 long and feathery, and thus are well adapted 

 to catch flying pollen. 



The flowers of insect-pollinated plants, on the other 

 hand, are usually showy, and have nectar or fragrance, 



