Februaky 1, 1001.] 



KNOWLEDGE. 



27 



siipport-s several large leaves, and a number of heavy 

 umbels composed of hundreds of ilowers. Their weight 

 is considerable, and as the leaves and umbels ai-e often 

 raised above the level of the surrounding herbage, 

 w-ind-pressure is also a serious item to bo provided for. 

 The stems of these plants are upright hollow columns, 

 strengthened by transverse partitions at intervals. Given 

 a certain amount of material a hollow tube is the 

 strongest form into which it can bo moulded to resist 

 pressm-e and bending. The plant has adopted precisely 

 the form in which the building materials which have 

 during the winter been lying ready stox'cd up in the 

 fleshy roots can be used to the greatest advantage. 

 These heavy umbels must be kept with their flat surfaces 

 horizontal, else the plant would lose the striking advan- 

 tages which this complicated inflorescence offers: hence 

 the necessity of rigidity. In many other plants with 

 tall annual stems, the form of inflorescence is such that 

 a temporary or permanent bending of the stem will 

 not render the flowers less conspicuous, or otherwise 

 interfere with their proper fertilization ; in these a 

 rigid stem is less required, and by bending to the storm 

 the plant will lessen the chance of accident. Compare 

 then with the stout I'igid hollow stem of the Angelica, 

 with its flat umbels of flowers, the thin flexible stems 

 of the Meadow-Sweet, Purple Loosestrife, or Meadow- 

 Rue that gi-ow with it, all of which bend to the breeze 

 that sweeps across the fields. The stems of water- 

 plants offer a different set of conditions. Just as their 

 roots serve as anchors rather than as gatherers of food 

 and water, so their stems act as cables rather than as 

 conduits The plant is buoyed up by the surrounding 

 water, the stem has not to support its weight. It acte 

 as a tie rather than as a strut, and flexibility and 

 tensile strength replace rigidity and 2>ower to resist 

 compression. 



Climbing stems exhibit remarkable features which 

 can only be brieflv mentioned here. In plants like 

 the Br)-onies. Hop, Vetches, the stem is suppoi-ted at 

 many points by its convolutions, which embrace the 

 support .to which it clings, or by the leaf-tendrils which 

 fulfil the same office. The stem is thus relieved of the 

 task of supporting the weight of £he plant, and serves 

 chiefly as a conduit connecting with the root, and passing 

 up water to the parts above. In these climbers the stem 

 is thin, for sti-ength is unnecessai-y, and it is flexible 

 and tough, that it may easily follow the movements of 

 the supporting plants. But it is the growing parts of 

 such stems that exhibit such remarkable features. In 

 these the rotating movement that characterises almost all 

 the growing parts of plants attains a degree that is truly 

 sui-prising ; and along with this is developed an 

 amazing sensitiveness which caiLses the stems or tendrils 

 to bend towards and twine round any support with 

 which, in their incessant movements of rotation, or 

 circuinnutation, they come in contact. Thus the climber 

 progresses, feeling for and gi-asping eveiything that will 

 help it in its journey to the li^ht and air. 



In a large number of plants the axis or stem is 

 so compressed longitudinally, that, as we have hitherto 

 understood the term, it might be said to be %vanting 

 altogether. Look at the Lesser Celendino, the Dande- 

 lion, the Daisy, and a hundred other wild-flowers. The 

 stem is reduced to an exceedingly short root-stock, inter- 

 vening between the roots on the one hand, and the 

 point of emergence of the leaves and flowers on the 

 other. Of stems that creep or lie prostrate on the 

 ground, or burrow under the ground, there is a gi-eat 

 variety. Prostrate stems, like those of water-plants. 



have not to support the leaves and flowers, and cau 

 aff'ord like the latter to be thin and whip-liko. Such 

 are the stems of the Ground-Ivy and the Creeping 

 Jenny, which produce leaves, flowers and roots through- 

 out their whole length. The Strawberiy, several of 

 the native Cinquefoils, and other wild-flowers, exhibit 

 two kinds of stems — short upright stems, or " crowns," 

 which give off lateral prostrate stems or "runners" 

 often many feet in length ; the latter produce at various 

 points of their growth tufts of i-oots below, and loaves 

 and flowers above, which by the withering of the inter- 

 mediate portions of the stem become in their turn 

 separate crowns, to give off new runners. Subtcrranerai 

 stems may conveniently be grouped similarly into those 

 which produce leaves and flowers throughout their 

 length, or at intervals. In tho subterranean stem a 

 further modification takes place as compared with tho 

 erect stem. Most erect stems — and prostrate stems too 

 — are coloured green with chlorophyll, that they may 

 assist the leaves in the manufacture of plant-food. The 

 undcrgi'ound stem has no opportunity of doing this, 

 owing to the absence of daylight, and it is usually 

 white, or of the dull colours that most roots alTcct. 

 Underground stems have likewise little need of sti'ength, 

 except the quiet but well-nigh irresistible strength of 

 gi-owth, by which the apex of the stem forces its way 

 through the soil. Their suj-facc, too, being buried in 

 damp earth, is less exposed to heat and di7ness, and 

 need not guard against excessive evajjoration : hence 

 we find that underground stems are frequently brittle, 

 with a very thin epidermis or skin. These stems are 

 excellent places for the storage of food-materials, which 

 is the more necessary in such jjlants since, the stem 

 being below ground, the leaves and flowers have to grow 

 up often to a considerable height above the surface to 

 seciu-e a due amount of light and air, and perfect the 

 fiTiit ; hence subteiTanean stems are frequently thick 

 and flesh}' — look at those of the Butter-bur, for instance. 

 An extreme case of the storage of food in stems is 

 found in tubers, such as the potato. In these, a great 

 amount of food-material is stored around a few buds, 

 which lie dormant during the winter, and use the food- 

 store in their rapid growth during the following season. 

 Stems may altogether .supplant leaves, and undertake 

 the manufacture of the whole of the food of the plant. 

 The Gorse funiishcs a well-known example. Tho seed- 

 ling Gorse has little trifoliate leaves like the Genistas, 

 to which it is related, but as the young Gorse inoreiases 

 in size these leaves disappear, and the green stem.s- 

 carry on the work of Ic-aves, and in addition under- 

 take the defence of tho plant against grazing animals 

 by means of the stout thorns in whicli the branches 

 terminate. ThLs principle is sometimes carried further, 

 and the stem becomes flattened and leaf-like, tho better 

 to carry on the work of assimilation. Tho Butcher's 

 Broom supplies one of the most marked instances of 

 this to be found among British plants. That tho leaf- 

 like organs of thLs plant are really stem-structures is 

 rendered evident by the fact that the minute flowers 

 of the plant are borne on their surface. Tho Duck- 

 weeds likewise furnish excellent examples of leaf-like 

 stems. 



Just as the sensitive i-oot-tip bends downward fi-om 

 the commencement of growth, so the stems of most 

 plants grow towards the light. In most plants this 

 bendingr towards the light, or heliotropi«rii, is more 

 powerful than their apogeotropiem, or tendency to bend 

 away from the direction of the force of gravity. This 

 is well seen when plants grow in a cave or recess; their 



