132 



MINUTE STRUCTURE OF THE STEM. 



leaves. It is known as the Phyllophore. The newest leaves are 

 formed nearest the apex of this cone ; and here, as before shown, 

 all the fibro-vascular bundles common to the leaves and stem ormi- 



o 



nate. In most cases there is absolutely no increase in thickness 

 of the stem below the base of this cone ; but as the apex of the 

 cone is developed and extends further upwards, thus elongating 

 the stem, there is also a growth in thickness of the part of the 

 cone just above its base. Thus a uniform size of the cylindrical 

 stem is kept. But such increase in thickness cannot continue 

 below the point at which there are active leaves. 



increase iu size, the number of bundles grows larger, and conversely. The 

 number of internodes through which a bundle passes cannot be fixed with 

 exactness. 



"Those bundles in a leaf-trace which curve like a bow towards the middle of 

 a cylinder do not penetrate to equal depths ; as a general thing, the median 

 bundle of a series lies deepest, and the others lie less deep in proportion to their 

 distance from this ; the marginal ones descend nearly perpendicularly in the 

 outer surface of the cylinder. "Where there are several series of bundles, those 

 in the inner series generally penetrate more deeply than those in the outer 

 ones which lie at an equal distance from the median thread. 



" The necessary consequences of the course described are : first, that in the 

 cross-section of an internode the bundles .stand closer together in proportion as 



they are nearer to the outer 

 surface of the cylinder. - 

 a phenomenon which is 

 especially noticeable when 

 the bundles are distributed 

 over the whole surface of 

 the cross-section of the 

 cylinder ; second, the suc- 

 cessive traces dwindle, and 

 their curving threads cross 

 each other. Mohl's cele- 

 brated plan, which is here 

 reproduced in Fig. 108, ex- 

 hibits this latter relation 

 in a radial longitudinal 

 section, being based on 

 the untenable assumption 

 that all the threads of a 

 trace are nearly equally 

 curved, and are placed in 

 a tangentially perpendicu- 

 lar direction, so that they 

 form in the outer surface 

 an open curving cone. If it 



108 



10'J 



FIG. 108. Mohl's diagram of the course of the fibro-vascular bundles. 

 FIG. 109. Diagram of the course of fibro-vascular bundles in ;i palm-stem with dis- 

 tichous leaves. (De Bary.) 



