SCIENCE- GOSSIP. 



199 



was on the point of netting a male, a female 

 collided with him, knocking the male on one side 

 and herself falling to the ground. She imme- 

 diately commenced crawling up the short grass 

 stems, the male hovering lower and lower towards 

 her, and in a very brief space of time the pairing 

 was complete. I saw many others act in a 

 similar manner, and in some of my former notes 

 elsewhere, I have stated that the females collided 

 with the males before pairing. Whilst in the 

 Hebrides closer observation during the past 

 season, however, convinces me that the collision 

 is due to accident rather than design, the object 

 of the female being to get immediately to the 

 windward of the male. She then alights on the 

 stems of grass below the male, and the pairing is 

 soon effected. The males after emerging from the 



pupae never go more than a few yards from the 

 place of pupation, though it may be a week or 

 more before a female comes that way. On the 

 evening following the pairing the male moth just 

 gives one or two pendulum-like swings, and then 

 flies away to any high point in his immediate 

 neighbourhood, preferably to trees ; or if there are 

 not any, to a projecting rock, or even a chimney- 

 pot on the top of a house. I think my observations 

 tend to show that, in the habits of the ghost- 

 moths, there is exhibited the unusual feature of 

 the females searching for males during the pairing 

 sea-on. This is probably the correct solution for 

 explaining the i - emarkable habit of flight in some 

 species of the Hepialidae. 



35 Averill Street, Fidham Pa 7 a"e Road, 

 London, W., October 1901. 



THE STRUCTURE OF PLANTS. 



A CHAPTER IN ORGANIC EVOLUTION. 



By Rudolf Beer. 



(Continued from page 141.) 



TTTE must in this article sift out the new condi- 

 * * tions which are the concomitants of a 

 terrestrial life, and so read the structure of the 

 higher plants in changed environment. 



The highest Water plant that we have thus far 

 studied was no more than a mass of cells all similar 

 to one another. Such a mass of cells could assume 

 various external shapes. , The cells might be 

 arranged as a long central' axis corresponding to a 

 stem, and upon this lateral organs, such as leaves, 

 might be borne. This separation of stem and leaf 

 is an arrangement through which the greatest 

 amount of surface is spread out to the rays of sun- 

 light ; but we can return to this point later, when 

 we will touch upon its importance to the plant. 



An example of a plant of this kind will be found 

 in Chara or Nitella, or, perhaps more correctly, in the 

 probable ancestors of these already complex forms. 



On land a plant of any size, with this configura- 

 tion and structure, would collapse and huddle 

 together the leaf -surface,* which can only be of full 

 use to the organism when it is expanded to the 

 rays of light. One has only to remove from the 

 water a plant of Callitriche, which is in this respect 

 similar, although otherwise a far higher growth 

 than the stage we are considering, to realise how 

 impossible it would be for this plant to attain any 

 size upon land, without at the same time losing a 

 great deal of the efficiency of its leaf -surf ace. The 

 manner in which this preliminary difficulty of a 

 water plant becoming adapted to the conditions of 

 the land was conquered was by the alteration of 

 certain of its cells. 



In the plant with its cells all alike these were 

 thin-walled, round, square, or perhaps oblong 

 structures. We had better at once learn to speak 

 of such cells as parenchyma cells, and remember 

 that they are the primitive form of cell-structure. 



In the plant, struggling with the conditions of 

 a terrestrial existence, certain of these parenchyma 

 cells developed greatly thickened walls, and at the 

 same time they became elongated in shape. It 

 was the ring of cells lying just below the surface 

 of the stem which underwent an alteration of 

 this kind and gave rise to a cylinder of mechanical 

 strengthening tissue. This stage is very well 

 illustrated in the structure of the bog mosses or 

 Sphagnums which carry on an amphibious life. 



Every cell must contain a sufficient amount of 

 water to maintain itself as a living object, and the 

 greatest difficulty «hich undoubtedly confronted 

 the earliest terrestrial forms was that of retaining 

 this moisture in adequate amounts. The organism 

 with a thin-walled cellular structure would rapidly 

 lose its water by evaporation, and the only creature 

 that could succeed under the new conditions 

 would be one which had the walls of the outer 

 cell-layer more or less impermeable to water. 

 Such an impervious outer layer covering (he 

 surface of the most external cells like a skin may 

 be found in all land plants of the present day. It 

 is called the cuticle, and the outermost layer of 

 cells of the plant which bears this cuticle is termed 

 the epidermis. While the plants remained of small 

 size this cuticle was amply sufficient for the pur- 

 poses of water-retention, but as they grew larger 



