STRUCTURE OF THE PLANT 23 



then gradually becomes again more distinct, appearing 

 this time in another form, as another organ, and so on : 

 the one replaces the other, the one passes imperceptibly 

 into the other, until the whole cycle of development is 

 closed and the primary and original organ reappears. 

 So far we have had only the leaf organs in view, but 

 beside them the body of the plant reveals two other 

 organs, the beginnings of which are to be found already 

 in the seed : these are the stem and the root, the structures 

 which support the leaves. These two organs, apparently 

 so different and growing in different environments, are 

 in some rare cases, however, capable of transforming 

 themselves into each other : the stem sinks into the soil 

 and assumes the character of the root, or the i^oot grows 

 up into the air, covers itself with leaves, and assumes 

 the character of the stem. Hence the stem and the 

 root, forming the axis of which they are the two modified 

 forms adapted to different conditions of existence, and 

 the appendage of the axis — the leaf — with its manifold 

 variations (scales, petals, stamens, and so forth) are 

 the fundamental external organs produced by a normally 

 developed plant during its life-time. 



In accordance with the general conception of the life 

 of a plant we have thus far taken it for granted that it 

 begins and ends with the seed. Doubts, however, arise 

 as to our right to attribute the origin, the real starting- 

 point of the life of a plant to the seed. May we not 

 perhaps go further back and find out its ultimate origin ? 

 For the seed we have been describing is still a very 

 complicated body ; we find in its embryo a com- 

 plete little plant with practically all its parts already 

 developed. 



In order to discover this simplest starting-point of 

 plant life we must turn to plants which are exceptions to 

 the general rule of the typical plant with seeds and 

 flowers, which we have just been considering. 



