648 DEFINITION AND CLASSIFICATION OF STEMS. 



necessary building materials, may all be distinguished from each other; but the 

 embryo itself shows no segmentation into axis and leaves. It looks to the naked 

 eye like a filamentous, spirally-rolled structure, which breaks through the envelope 

 of the seed-coat on germination, extends and elongates, then grows up straight and 

 afterwards twists and winds and seeks for a resting-place from which it can derive 

 nom'ishment. This thread may, without further discussion be considered as a stem 

 although it beai-s no leaves, and indeed never presents even the rudiments of leaves. 

 Not until later, when this thread-like stem has developed haustoria at the spots 

 where it is in contact with the host-plant, and has grown still longer by the help 

 of the absorbed nourishment, do small scales which must be interpreted as leaves 

 arise below the growing point (c/. fig. 35 ^ on p. 175). Projections are then developed 

 above the scales which grow out into lateral shoots. 



The fact that stems exist, which, in their young condition exhibit neither leaves 

 nor even the rudiments of leaves, is specially emphasized here, because it has been 

 repeatedly denied that the stem is a special member of the plant. This may of 

 course seem strange to the uninitiated, and it will be asked, how then are we to 

 regard the stem if it has not the value of an independent morphological member? 

 Although this theme is so delicate, and its treatment so difficult for those who are 

 not initiated into the details of the speculative science of foi'm, yet I will try to 

 briefly state the grounds which have led to the opinion stated above 



At the free extremity of a growing leafy shoot a slight difi'erence maj^ indeed be 

 recognized between the cells of the periphery and those of the interior, but no clear 

 boundary can be fixed between these parts, and the end looks like an undifferen- 

 tiated conical or hemispherical mass of tissue. On observing more narrowly the 

 growth and further development of the mass, it will be noticed that cushions or 

 protuberances arise on the periphery of the cone and form leaves, while the inner 

 portion above these leaf-rudiments continues to elongate as an undifferentiated 

 mass. Soon, however, fresh rudiments of leaves arise from it, and so as the process 

 continues quite a large number of the cells are grouped together and form in their 

 turn the starting-points of leaves. If we examine the tissue of a leaf as it arises 

 thus below the tip of the shoot, we shall seek in vain for the place where the 

 substance of the leaf ceases and that of the stem begins. It is on such grounds as 

 these that the view has been formulated that the whole stem is really nothing else 

 than a collection of leaves, standing one above the other, whose basal portions 

 remain united, while the peripheral parts according to need rise up and project 

 more or less. Against this view, of course, there is apparently the fact that not 

 only leaves but also lateral shoots appear on the circumference of a growing shoot, 

 from which it follows that the whole of the tissue is not employed in the formation 

 of leaves, but that a part remains over from which the commencements of lateral 

 stems ai-e produced, and that it is this part which does not form leaves which repre- 

 sents the tissue of the main stem. It has also been proved that the rudiments of 

 leaves arise on the growing shoot-cone from cells lying nearer the periphery than 

 those from which the rudiments of lateral stems de^'e]op. This different origin has 



