GROWTH OF VASCULAR BUNDLES 51 



take part in the formation of a sieve tube, while one facing 

 inward undergoes quite other transformations to form a tracheal 

 tube ? And how can the daughter cells facing inward become 

 any one of the xylem elements, apparently quite at w r ill ? They 

 all have the same parentage, and probably the same potentialities, 

 but behave differently, possibly because they are responding to 

 stimuli of different natures. It may be that these stimuli arise 

 in present need or necessity; but the response is modified by the 

 peculiar potentialities of the particular species. Thus, the daugh- 

 ter cells of the cambium in the oak, feeling the need of more 

 water-transporting tissues, would transform themselves into 

 tracheal tubes, while in the pine, under like circumstances, 

 tracheids with large cavities would be formed. The so-called 

 ring of growth throws some light on this question and will now 

 be considered. 



In the stems and roots of trees and shrubs it is found that 

 the addition made to the xylem during each growing season 

 consists of two more or less well-defined parts, namely, an 

 early growth, in which the tracheal tubes are relatively more 

 abundant and possess larger cavities (Fig. 24), or w r here, as 

 in conifers, the tracheids have relatively large cavities and thin 

 walls; and a late growth, in which the tracheal tubes are rela- 

 tively fewer and smaller, and the tracheids have smaller cavities 

 and thicker walls (Fig. 52). In the early growth the water 

 conduction elements may be said to preponderate, and in the 

 late growth the strengthening elements; and this is as it should 

 be, for the plant first feels the need of water in the spring, and 

 later the need of greater strength. The first manifestation of 

 growth in the spring is the unfolding of the leaves, and in Dicoty- 

 ledons and Gymnosperms there are more of these than appeared 

 the previous year, for the crown of branches grows larger every 

 year; and even if the old tracheal tubes or tracheids were in direct 

 communication with the new leaves they would not suffice. 

 But the old water channels in the stem do not extend into this 

 year's leaves and new ones must be formed which will be contin- 

 uous with those in the leaves (see Fig. 25). Later, when most 



