600 BOTANY OF THE LIVING PLANT 



point are approached from below the control is relaxed : in passing finally to 

 the procambial region a point is reached where outward diffusion from the 

 stele is no longer so restricted : the distal growing point may thus be supplied 

 with water and solutes by upward diffusion over the whole cross-sectional area. 

 This may roughly be compared with the spout and perforated " rose " of a 

 watering pot. In either case the separate streams merge at last into a common 

 supply to the region immediately below the growing point. 



The apical structure of Palms and other Monocotyledons corresponds func- 

 tionally with that in Ferns, though the strands differ in the former by the absence 

 of endodermis surrounding each strand (Fig. 31). In both the existence of a 

 medullary vascular system may find its explanation in the need for bringing 

 adequate supplies towards the centre of a progressively expanding distal bud, 

 in plants of primary construction, where the apical bud and leaves are large, and 

 are served by stems without secondary increase. 



Large and Small Buds Compared. 



We may now return to the opening of this discussion on p. 589. In 

 the words of Herbert Spencer, green plants are accumulators of 

 material gained by photosynthesis. They are in no equivalent degree 

 expenders. Hence they naturally work towards a favourable balance 

 of material. One consequence of this is a prevalence of primary 

 expansion upwards, and the development of large distal buds : as seen 

 in Ferns and Palms. Here the medullary vascular tracts may serve for 

 the transit of nutrients through the inert pith towards the centre of 

 the apical region, with its acropetal succession of young leaves. In 

 fact the conducting systems of Ferns and Palms, etc., on the one hand, 

 and of woody forest trees on the other, have solved their respective 

 problems of advancing size and internal structure in quite different 

 wavs. In the former, there has been adherence to their original scheme 

 of primary obconical expansion of the shoot with dilation of the stele in 

 relation to the single, enlarging terminal bud : in the latter, what is 

 probably a more efficient method both for mechanical support and 

 for supply has been evolved ; that is, for the development of a multi- 

 tude of small buds that initiate a profuse branching. In these the 

 vestigial relics of the old primary wood translocate a sufficient supply 

 to meet the earliest demands of each small bud ; but this is supple- 

 mented without delay by continued secondary increase through 

 cambial activity which meets both nutritional and mechanical needs. 

 The final result of this difference in solving the fundamental pro- 

 blems of Size may be seen in any mixed forest, where conical woody 

 plants with small buds are dominant, and form the canopy beneath which 

 the obconical and big-budded types shelter. 



