BY D. MCALPINE. 661 



innumerable plume-like branclilets are given off, ending in 

 their delicate intertwining tufts immediately beneath the 

 skin. Here a new supply of food-material is provided for, for 

 the green chlorophyll-containing cells of the hypodermal layer 

 (Fig. 12), in the presence of sunlight, manufacture starch, 

 which is converted into sugar, and carried by the plume-like 

 branches to the enveloping network, where it meets and blends 

 with the ascending stream. 



As regards the course followed by the food-materials, the 

 most generally accepted theory is that the carbohydrates and 

 the proteid substances follow two separate paths in their 

 passage from the place of formation, the one through the 

 " conducting parenchyma," and the other by means of the 

 sieve-tubes. 



But the view that these substances are exclusively conveyed 

 in particular tissues is becoming modified, and Sachs, in his 

 " Lectures on the Physiology of Plants," states at p. 358, " In 

 the case of a very vigorous transport of starch, as when the 

 leaves of trees are being emptied in the autumn, even the 

 phloem of the vascular bundles may take part in it." 



The theory put forward by Czapek, in 1897, that the car- 

 bohydrates are transported by the sieve-tubes, is now gaining 

 ground, particularly when they have to be carried for some 

 distance. There are thus two possible ways in which food- 

 materials may be transported — by the slow process of diffusion 

 from cell to cell, when the solutions in each have different de- 

 grees of concentration ; or more rapidly by means of the sieve- 

 tubes, with their greater length, and pores in the transverse 

 walls. 



Vascular hundles in relation to each other. 



It has finally to be noted, that the primary vascular bundles 

 are not isolated from each other, but that they anastomose at 

 various points. The branches which supply the core and the 

 flesh ultimately form a network, the one enveloping each car- 



