8 THE FLOWERING PLANT. 



functions. And again, different members may be similar organs, 

 and hence display analogy. Thus, among climbing plants, the 

 ivy climbs by its roots, the convolvulus by its stem, the pea by 

 modified leaves, and the hop by both stem and hair- structures. 



Structure of Plants. — Careful examination of plant-members 

 shows that they are not homogeneous in structure. A thin skin 

 can be peeled from the upper and lower surfaces of most leaves, 

 and then comes a green pulp, traversed by firmer strands, popu- 

 larly called nerves or veins. They are the parts which make 

 up " skeleton " leaves, the softer structures having rotted away. 

 These different components of leaves, (fee, are called tissues, and 

 it can be shown by the microscope that they, in their turn, are 

 made up of smaller parts known as cells and cell-derivates. These 

 cells, which are the units of plant-structure, just as bricks and 

 stones are in architecture, are for the most part microscopic, 

 though sometimes of large size. The pulp of an orange, for 

 example, is made up of such large cells, resembling in this case 

 spindle-shaped bags, and containing a good deal of fluid. Botani- 

 cal knowledge is based on the structure and physiological powers 

 of cells, and the very simplest plants known (and simplest animals 

 too) are single cells, in other words, they are unicellular. 



Life under Simple Conditions. — Without reference to any 

 special example, we will consider the structure and conditions 

 of life in one of these simplest forms, and then see what modifi- 

 cations exist in multicellular plants, i.e., those made up of many 

 cells. 



The general form of the body in our ideal example is spherical, 

 and this appears to be a common result of uniformly distributed 

 external influences. The shape is distinct and permanent, owing 

 to the presence of a firm elastic membrane, the cell-wall, which 

 forms a kind of superficial skin. Within the wall, and forming 

 a lining to it, comes a slimy layer, the protoplasm, and part of 

 this is of different texture to the rest, and forms a firmer round 

 or oval nucleus, which again contains a nucleolus. There are 

 also present a number of granules of dense protoplasm, permeated 

 with chlorophyll, a green colouring matter. These are chlorophyll 

 granules. The larger internal space or vacuole is full of fluid, the 

 cell-sap, which consists of water holding various substances in 

 solution. It is also supposed to permeate the protoplasm and cell- 

 wall. A cell from the pulp of an orange corresponds fairly well 

 with this description, but a slight modification of shape has taken 

 place. So much for the morphology. The most essential phy- 

 siological fact with which to start is that the protoplasm is the 

 only living part of the cell. Very young cells contain no vacuole, 

 which is produced later on, owing to the fact that growth in 



