200 



SCIENCE- GOSSIP. 



and the stem longer the danger of a too great loss 

 of water constantly increased. The barriers of 

 the cuticle would be gradually forced, and too 

 much water lost in its passage through the stem 

 to suffice for the needs of the growing apex. To 

 meet this danger certain layers of cells lying 

 beneath the surface underwent changes in the 

 character of their cell-walls, similar to those we ■ 

 have seen in the cuticle, which rendered them, 

 each and all, impermeable to water. Layers of 

 cells that have become modified in this way are 

 called cork, and, as is familiar to you, this tissue is 

 removed from the plant and put to use in our daily 

 life on account of its specific peculiarity. Being 

 impervious to water, the cork cuts off any layers of 

 cells which may happen to lie externally to it 

 from the water that is essential to them and which 

 is moving within the stem. Consequently these 

 outer cell-layers die, and their dried remains, 

 together with the cork, form the well-known bark 

 of trees. 



If the entire surface of the plant were enveloped 

 in a coat through which moisture could not pass it 

 would be difficult to see how many processes, im- 

 portant for the maintenance of plant life, could 

 continue. 



We find, however, that this water-tight casing of 

 the plant is broken at two points : one below where 

 the base of the plant, the root, buries itself in the 

 moist earth ; the other above at the opposite 

 extremity of the plant. 



The lower passage for water is, as we have said, 

 the root. The particular spots of the root through 

 which water is found to enter the plant from the 

 usually damp soil are the root-hairs which cover 

 the younger portions of that organ in abundance. 

 The second channel for water above is furnished 

 by the leaf -pores or stomata. Experiment shows 

 us that water is leaving the plant from these 

 organs. There is, therefore, a constant stream 

 of water passing through the plant from below 

 upwards. Along what channel and how does 

 it travel ? The second part of this question 

 is to a great extent a dark and hidden mystery to 

 us. That the force of evaporation through the 

 stomata is largely concerned in the process we 

 cannot doubt; but it is, by itself, insufficient to 

 raise water to the great heights attained by several 

 forest trees. Many attempts at an explanation 

 have been made, but none are entirely satisfactory. 

 We are more certain with regard to the path 

 taken by the water in its journey through the stem. 

 In the earliest terrestrial forms the cells forming 

 a central column running through the heart of the 

 stem became greatly drawn out in length, while at 

 the same time they retained their thin permeable 

 walls. Along such a column of elongated cells 

 water can travel easily and rapidly. We see a 

 conducting column of this simple nature in the 

 stems of the majority of the mosses. 



As the struggle for life n ged among the early 



plants this simple conducting arrangement was 

 more and more elaborated. The thin-walled fibres 

 which we see in the moss thickened their walls, 

 without at the same time losing their permeability 

 to water. 



Moreover, to facilitate the passage of water from 

 cell to cell a curious system of thin spots was left 

 on the otherwise thickened wall. These thin areas 

 on the wall are called " pits," and form a most 

 characteristic feature on the conducting fibres. 

 Beautiful representations of pitted fibres are to be 

 found in ferns and in conifers. 



In higher development still those walls which 

 separate the cavities of fibres standing above one 

 another in rows are absorbed, and long pipes or 

 vessels are formed. In the higher plants we, 

 therefore, find a system of pipes — miniature water- 

 pipes — along which the wafer can move without 

 hindrance of any kind. 



It should be mentioned that both in the con- 

 ducting fibres and in these vessels the living con- 

 tents die and nothing remains that is functional in 

 the mature plant except the skeleton of dead cell- 

 walls. Any of the higher flowering plants, such 

 as an oak-tree or a sunflower, will furnish examples 

 of these vessels, and show us the delicately-pitted 

 markings on their walls. 



The green chlorophyll, which is so closely asso- 

 ciated with the nutrition of the plant, can only 

 carry on its functions when thoroughly illuminated. 

 In the body of a plant, consisting of a mass of 

 cells, it would be useless for chlorophyll to be 

 developed in the innermost cells, as these are 

 completely shaded by those lying outside. Any 

 structure or organ that is useless to a living- 

 creature involves a waste of energy and material 

 which would be fatal to the organism in its 

 inevitable combat with its fellows, and very soon — 

 that is to say, in a few thousand years — the need- 

 less structure would be eliminated. This explains 

 the fact that "no single hair," as an old naturalist 

 put it, " is without its definite use to the plant." 



According to this " law," as we might almost 

 call it, chlorophyll is only found on superficial 

 areas of the plant, capable of illumination by the 

 sunlight. 



Thin laminae or plates of cells containing 

 chlorophyll would form the most advantageous 

 arrangement, in which each green cell would be 

 assured of a sufficient supply of light, and in which 

 at the same time there would be a considerable 

 extent of surface working for the nutrition of the 

 plant. This ideal mechanism is realised in the 

 leaves of a plant. We have already seen that quite 

 early in the history of evolution a differentiation 

 of leaf and stem took place. The stem, we observe, 

 is a structure to support and spread out the leaves. 

 At first the stem was short and slender, but as 

 vegetation increased on land, and suitable spots 

 become thickly garmented with low growing herbs, 

 the search for light became increasingly more 



