1250 



FLOWER 



FLOWER 



1537. Wind-pollinated flower 

 of juncus. (Enlarged) 



tain starch as a reserve food instead of oil. This pol- 

 len is capable of withstanding greater desiccation than 

 is the pollen of most insect-pollinated flowers. In the 

 pines, each grain is provided with two air-sacs to 

 increase the buoyancy and to expose greater surface 

 to the wind. 



Insect-pollinated or ento- 

 mophilous flowers must meet 

 two distinct problems: they 

 must entice the insect to the 

 flower; and they must guide 

 the insect in such a way that 

 cross-pollination will be as- 

 sured. The attractive agents 

 are four in number, color, 

 honey, scent, and abundant 

 pollen (for pollen - eating 

 insects), but they are not 

 usually all found in one 

 species. Color is provided 

 mainly by the corolla, but 

 the calyx (in Anemone) or 

 even the bracts around the 

 flowers (in Cornus and Poin- 

 settia) may function thus instead. Attempts have 

 been made to show that certain colors are more attrac- 

 tive than others to certain groups of insects. Yellow 

 has been designated as the color for flies and beetles, 

 blue and red for hymenoptera, browns for carrion 

 insects and wasps, and whites for night-flying insects 

 especially. Honey (nectar) is produced in a great 

 variety of flowers and it is a reward for the insect visit. 

 The honey-secreting glands (nectaries) are borne either 

 on the disk or on the petals, but more rarely are they 

 staminal or ovarian. In order that the honey may not 

 be appropriated by undesirable insects which would not 

 effect cross-pollination, it is frequently placed at the 

 end of spurs or grooves which are adapted to the pro- 

 boscis of the insects for which the flower is adapted. 

 Various markings of the corolla, such as bright eye- 

 spots and dark converging lines, called honey-guides, 

 often direct the insect accurately to the honey, and in 

 such a way that cross-pollination will be accomplished. 

 An interesting case is the violet, where the honey is 

 produced by staminal nectaries but is collected and 

 stored in the spur of the lower petal. To this storehouse 

 honey-guides in the form of purple lines lead. The beard 

 in the throat of the violet flower protects the pollen 

 from rain and also discourages the insect from entering 

 the flower on the wrong side. Scent as a means of 

 attracting insects is very general, and is especially 

 frequent in nocturnal and crepuscular (twilight) 

 flowers. The scent is due to 

 volatile oils produced mainly by 

 the petals. These oily compounds 

 are comparatively few in number 

 and often re-occur in plants that 

 are wholly unrelated. Thus the 

 clove scent is found also in some 

 orchids, and the violet scent is 

 found with slight modification in 

 the flowers of several plants. Flow- 

 ers that attract pollen - eating 

 insects are often yellow, as butter- 

 cups and dandelion, but flowers of 

 other colors are frequently visited 

 at least by bees that carry away 

 quantities of pollen in their femoral 

 pollen-pockets. 

 Most pollen is injured by exposure to rain and dew. 

 The grains tend to swell and burst owing to the exces- 

 sive osmotic pressure. It is for this reason that pollen 

 when studied or germinated in the laboratory must be 

 mounted in a sugar solution approximating the density 

 of the stigmatic fluid. It is not a surprise, therefore, to 

 find that nature has protected the pollen of many 



1538. Wind -pollin- 

 ated flower of a grass. 

 Poa. (Enlarged) 



flowers from rain, by structural means. Thus, bell- 

 shaped hanging flowers, salverform corollas with a 

 small eye which requires pressure to force a drop of 

 water in, closed corollas of the snapdragon type, beard 

 in the throat, flowers that droop only in wet weather, 

 flowers that close up during rain, and many other con- 

 trivances, are adaptations, in part at least, for the pro- 

 tection of the pollen. 



The protection of the honey and pollen from unbid- 

 den insect guests and the safeguarding of the flower 

 from self-pollination by such insects, has led to various 

 protective devices. The closed throat of the toadflax 

 and snapdragon, the small eye of the salverform corolla, 

 the beard in the violet, setose peduncles and stems over 

 which insects can walk with difficulty, glandular pedun- 

 cles and bands of viscid matter which serve as a sort 

 of sticky fly-paper to prevent wingless insects from 

 reaching the flower, are all adaptations of this nature. 

 Remarkable in this respect is the teasel, which has 

 connate-perfoliate leaves. These leaves form a basin 

 around the stem at each node. The basins fill with 

 water during each shower, and, as the water will not 

 evaporate for several days, there is a veritable moat 

 around the stem at each node which climbing insects 

 cannot pass. 



Cross-pollination is frequently rendered more cer- 

 tain by various mechanical devices. Thus a device of 



1539. Dimorphic flowers of primula. 



great efficiency found in many plants is the separation 

 of stamens and pistils in different flowers (diclinism) 

 which renders self-pollination impossible. In this 

 respect, the dioecious plant is the most perfect type. 

 Diclinism is especially common in anemophilous plants, 

 in which the pollen is blown about indiscriminately. 

 Another efficient device consists in the early matura- 

 tion of the stigmas (proterogyny) or of the stamens 

 (proterandry) before the other sex in the same flower 

 (condition of dichogamy). Still another, although much 

 less common device, is the production of two or three 

 types of flowers in the same species in which the styles 

 and stamens are of different lengths (heteromorphism) . 

 Thus in the primrose (Fig. 1539) one flower may have 

 long stamens and short style, and another flower short 

 stamens and long style (dimorphic), so that an insect 

 coming from a long-stamened flower will have pollen 

 on his proboscis at exactly the right height to brush 

 the stigma of the long -sty led flower. In Ly thrum 

 Salicaria, the various combinations between the length 

 of style and of each of the two sets of stamens furnish 

 three types of flowers (trimorphic) . Other devices are 

 often found. Thus in some flowers the pollen of another 

 plant is prepotent in fertilization over that of the same 

 plant if both are placed on the stigma at the same time. 

 There are also many special structural mechanisms in 

 individual species, a study of which forms one of the 

 most interesting chapters in biology. Here may be 



