Il6 ANATOMY OF THE GYMNOSPERMS 



In Sequoia and Abies we have two genera which are remark- 

 able for their transitional forms of structure, affording a fairly 

 clear conception of the genesis of the resin passage. In each 

 case there is a well-defined tendency toward the aggregation of 

 the resin cells into compact groups which take the form of lon- 

 gitudinal strands, inclosed on all sides by the accompanying 

 parenchyma tracheids. Under such circumstances the individual 

 cells undergo a continual reduction in length until they eventu- 

 ally become but two or three times longer than broad, or they 

 may even become isodiametric. This change is not accompa- 

 nied by any alteration in the thickness of the walls in the earlier 

 stages of development, but as a result of such a shortening the 

 effect is to bring about the concentration of a greater number 

 of simple pits within a given area. Such cells, therefore, are 

 always more strongly pitted than those which are isolated and 

 of greater length. When aggregates of this sort have attained 

 to a certain degree of development a line of cleavage arises in 

 the center of the mass and results in the formation of an inter- 

 cellular space which, according to Eichler (15), always arises 

 schizogenously. This space is short and either isodiametric or 

 but little longer than broad, the length coinciding with the prin- 

 cipal axis of growth. Such cystlike reservoirs or sacs represent 

 the primitive form of the resin canal, and they are typically 

 developed in Sequoia, Abies, and Tsuga. They always form a 

 continuous series extending in a direction parallel with the axis 

 of growth ; but as the type of organization advances, they merge, 

 forming a continuous canal such as may be found typically in 

 Pseudotsuga or Pinus. From these statements, then, it is clear 

 that the parenchymatous resin cells undergo modification in two 

 directions, passing into parenchyma tracheids on the one hand, 

 and on the other becoming shorter and shorter, according to 

 conditions of aggregation, until they pass into short cells which 

 eventually constitute the epithelium structure of the somewhat 

 complicated resin passage, the latter thereby becoming the ex- 

 pression of a peculiar aggregation of resin cells. Whatever 

 the stage of development may be, the resin passage is always 



